Packs that include a second pack to use as a day pack have that feature highlighted in their descriptions, since are getting two packs for the price listed and should take that into account when making your decision.

Recommendations based on what I personally use:
These are the packs most similar to the one I own and use for my trips

Eagle Creek Rincon 65L, price: $260 plus FREE shipping at eBags 45 L main pack + 20 L daypack, 25.2" x 9.65" x 13.39", weighs  5 lbs, 8 oz., full front panel loading, central lock point, zip-off day pack, compression straps, zip-away suspension for baggage check, and a customizable suspension for ultimate comfort and fit over the long haul. 

Eagle Creek Rincon Vita 65L Women's, price: $260 plus FREE shipping at eBags 45 L main pack + 20 L daypack, 22.05" x 9.06" x 13.78" , weighs  5 lbs, 8 oz., women's contour fit, full front panel loading, central lock point, zip-off day pack, compression straps, zip-away suspension for baggage check, and a customizable suspension for ultimate comfort and fit over the long haul.

Recommendations based on price & backpack features:
I only list packs that I would consider using myself based on the criteria I discuss here. Reviews of the backpacks are available on some sites, like eBags, so read those to help you make a decision.

 
SOC Gear Bugout Bag, price: $72.99 at eBags plus FREE shipping at eBags 2700 cu. in., 22" x 15.5" x 8", 5 lbs, 12 oz. Expandable (by 3") clam-shell main compartment with compression straps provides for easier packing and quick access to stored items.  Personal Comments: For the size, this bag is a bit on the heavy side (compared to its carrying capacity) but it is a cheap price and meets my basic travel backpack criteria. Read the reviews at eBags for more info.  NOTE: This pack is only meant for someone prepared to travel really light.
 
Osprey Porter 46, price $99 at eBags plus FREE shipping 2807 cu. in. (46 L), 22 x 14 x 9 in., weighs 3 lbs, 2 oz. easy-stow harness and fabric wing hipbelt, internal organization, and carry handle. NOTE: This pack is only meant for someone prepared to travel really light.

Kelty Redwing 50 M/L, price: $109.99 at eBags plus FREE shipping 3,100 cu. in., 25" x 18" x 14.5" , weighs 3 lbs, 8 oz . Panel loading. Removable Waist belt. Zippered side pockets and mesh water bottle pockets. Large front pocket with organization. NOTE: This pack is only meant for someone prepared to travel really light.

Mountainsmith Alder 30 Women's, price $119 plus FREE shipping at The Backpack Travel Store  2563 cu. in / 42 L, 11.75" x 22" x 9", weighs 3.4 lbs. Shoulder and hip straps designed to fit a woman, opens like a zippered suitcase for easy access, contoured lumbar support, muliti-compression straps, external water bottle pockets and super eco-friends since it is made from recycled water bottles. NOTE: This pack is only meant for someone prepared to travel really light.

Osprey Porter 65, price: $129 plus FREE shipping at Moosejaw 3900 cu. in, 26" x 15" x 10", weighs 3 lbs, 4 oz. Zip-away harness and stowable hip wings, internal mesh and fabric organization pockets, large easy access top pocket, lockable main zipper, straightJacket compression with foam crescent sidewalls, top & bottom and side carry handles. 

Caribee Short Hop 55, price $129 plus FREE Shipping at The Backpack Travel Store  3349 cu. in./55 L, 23.5" x 15" x 10", weighs 5lbs. Advanced hip and lumbar support, hip belt pockets keep essentials in easy reach, separate "laundry shoot " compartment keeps dry clothes away from clean, and concealable rain cover.

 
REI Vagabond Tour 40 Travel Pack, price: $149 plus FREE shipping at REI 38 L/2,318 cu. in., 24.5 x 10 x 7 in., weighs 3 lbs, 9 oz. Fits torsos 17-19 inches. Panel-loading access. Integrated cover encloses straps when pack is checked as baggage and doubles as a raincover for use in wet weather.  NOTE: This pack is only meant for someone prepared to travel really light.

Caribee Mallorca 70, price $155.95 plus FREE shipping at The Backpack Travel Store 70 L, 25.5'' x 13.4'' x 10.2'', weighs 6.2 lbs.  Opens fully like a suitcase for easy access, removable daypack, fully adjustable Vantage harness system ensures the perfect fit, concealable cover keeps straps stowed and safe in transit, internal divider can separate bottom and top compartments, lockable zippers for additional security, and separate shoe or dirty clothes compartment.
 
Osprey Farpoint 55, price: $179 plus FREE shipping at Moosejaw 3300 cu. in., 24" x 17" x 13", weighs 4 lbs, 1 oz. Two torso sizes to optimize fit, zip-away contoured shoulder straps and hip belt for outstanding fit, removable daypack for guidebooks, jackets and everyday use, daypack attaches on front main backpack straps when security is the issue, top and side carry handles, panel zip main compartment with lockable pulls, removable sleeping pad straps, internal compression straps, and large internal mesh pocket . 

Deuter Transit 50 Travel Backpack, price: $179 plus FREE shipping at REI 3,052 + 610 cubic inches/62 L, 25 x 13 x 10 in., weighs 4 lbs. 12 oz. Detachable daypack. Concealable backpack system zips away for airline transport or duffel/shoulder carrying. Load-bearing aluminum frame and anatomically formed hipbelt make carrying heavy loads comfortable.

Osprey Farpoint 70, price: $199 plus FREE shipping at eBags 4200 cu. in., 26" x 18" x 14", weighs 4 lbs, 3 oz. Two torso sizes to optimize fit, * Zip-away contoured shoulder straps and hip belt for outstanding fit, removable daypack for guidebooks, jackets and everyday use, daypack attaches on front main backpack straps when security is the issue, top and side carry handles, panel zip main compartment with lockable pulls, internal compression straps, and .arge internal mesh pocket.

Eagle Creek Adero 45L, price $215 at eBags plus FREE shipping 2750 cu. in., 24.41" x 8.66" x 12.99", 4 lbs, 8 oz. Full front panel and zippered top-load. Fixed top lid with zippered documents pocket and Secure-Zip Toggle closure. Central Lock Points secure main entry points.

Osprey Waypoint 65 Men's, price $249 at eBags plus FREE shipping 4000 cu. in./65 L, 28 x 15 x 14 in, weighs 5 lbs. 12 oz. Interchangeable Waypoint harnesses help you customize the fit while a built-in ErgoPull hipbelt keeps the load comfortably on your hips without being bulky. A detachable daypack with a padded front panel keeps your smaller items safe.
 
Osprey Waypoint 65 Women's, price $249 at eBags plus FREE shipping 4000 cu. in./65 L, 28 x 15 x 14 in, weighs 5 lbs. 8oz. Interchangeable Waypoint harnesses help you customize the fit while a built-in ErgoPull hipbelt keeps the load comfortably on your hips without being bulky. A detachable daypack with a padded front panel keeps your smaller items safe.

Eagle Creek Loche 70L Travel Backpack, price: $225 at eBags plus FREE shipping 4270 cu. in., 31.5" x 9.84" x 13.78", weighs 4 lbs. 14 oz. Full front panel load and zippered top access, multiple cary handles for easy grab-and-go, internal and external compression straps, and central lock points secure main entry points.

Deuter Traveller 55+10 SL Backpack - Women', price $279 at REI plus FREE shipping 3,356 + 610 cubic inches/65 L, 26 x 14 x 10 in., weighs 6 lbs. 8 oz. Detachable daypack. Concealable backpack system zips away for airline transport or duffel/shoulder carrying.

Still don't see the right pack for you? Check out my places to buy backpacks to find the right pack at the right price.

Wheeled Bags:
I don't recommend wheeled packs, as they are heavier than regular bags and impractical to use in Europe (cobblestone streets, lots of stairs, etc.). 

Still don't see the right pack for you? Check out my places to buy backpacks to find the right pack at the right price. 

Prices listed above are in US$. Price and availability of packs is subject to change (please let me know if the prices have changed or the packs are no longer available and I will correct my listings ASAP).