I need a good bag to make a few day trips through Europe. What's a good one?

Timbuk2?

Timberland “makes” a bag (Samsonite really makes it) that is super rugged, the size of a carryon (a rolly one) and transforms into a backpack. It’s all soft sides, has an expansion, and has a garmet bag. It’s wonderful! I’m using it for a 10 day trip to Australia later this year and on a weekend trip to NYC right now. It is often on sale for about $50.

Are you looking for an average rucksack or a hiking bag? Will you be carrying valuables in the bag? Fragile objects such as a laptop? If you’re searching for something to carry mostly clothes and toiletries I would suggest a simple “over the shoulder” sports bag; they’re cheap, have plenty of space, are convenient and rarely, if ever, break/tear.

whatever bag you choose, just make sure its comfortable to carry. The bag I used when traveling in turkey last year seemed durable, light enough – and, to be honest, I really liked the design. But while traveling I discovered that the bag straps were not very strong, it grew heavier with each day, and worse, cut into my shoulders.
I like nice bags, but for on the move travel, practicality is the best way to go.

Eagle Creek makes several styles. I have one that’s comfortable and durable. Also, Rick Steves website has one that’s nice and Patagoinia, but theirs are pretty spendy.

A lot of hostels have safes now. You probably don’t need to take all of your stuff with you when you leave. Take your camera & iPod, and leave your clothes & toiletries.
And, if you are worried about money, get a money belt.

You can visit a major sporting goods store and look at various styles. Seeing the bags in person is the best way to choose.

Baggalini makes yummy looking bags in many sizes and good colors. But I agree with @YARNLADY that you need to try them on. If that can’t happen, I do hear really good things about Rick Steves’ bags, even from people who don’t like his accent.

What type of bag are you talking about?