Unless you're a student, forget the backpack.

The recommendations for the Tom Bihn tristar, or the ebags weekender are good. I'm a big fan of going the lightweight one-bagger approach with these travel packs. However, I don't like the looks of them--they looks like something a college kid carries between youth hostels while traveling through Europe. Tumi bags are very nice indeed, but IMO they're a bit too expensive. My personal preference is the Victorinox line of luggage and my favorite model is the Victorinox Standard Issue. For me, it strikes the best balance between lightweight one-bagging needs and a bag that doesn't look out of place carrying it into a business setting (it just looks like a large laptop bag really).

If the Standard Issue is a bit too big, look at the Standard Brief. I use the Standard Brief as my daily carry to the office and I have used it as a small overnighter bag too. It's not convertible like the Standard Issue, but the file divider section is held by velco and can be detached to use that room for clothing and other misc travel items if desired.