We do a lot of trips.
I like to have as little stuff as possible.
I bring a small over-the-shoulder bag for 2-3 night urban trips,
a big Rubbermaid bin (or two) for car camping,
and borrow from my boyfriend a pack for my son's camps or for my 5-night hikes.
My son and I have coming up:
a winter camp which has specific baggage requirements imposed,
at least several weeks of summer car camping,
a one-month overseas trip (flight, car, transit),
several week-long retreat-style camps,
a few 2-3 night urban style gigs.
My son's youth organization requires him to have a pack large enough to fit a lot of stuff (sleeping bag, pillow, clothes for a week, etc), but small enough for him (9yo) to carry. Tricky! (I may finally get a stuff sack this year.)
I have used this one for various events:
http://www.deakin.com/shop/products/?p=8531 . I really like it, but it's not comfy for 5-night hikes. And I'd go for its duffle/knapsack combo so stuff is easier to access.
I'm eyeing this pack:
http://www.mec.ca/product/5031-342/black-diamond-demon-duffle-bag/ but it may be too small for bigger trips while too large for carry-on.
I prefer waterproof, horizontally loading, knapsack wear, cleanable, and preferably carry-on size (thought that's the last priority).
I take excellent care of stuff, so things tend to last me many, many years or be very resellable.
What works awesome for you? What are the most versatile options out there? How few packs can support a breadth of travel approaches?
