It depends on 2 things for me - who are you going with (by yourself or with guys/gf) and what kind of things do you like doing when in a city - visit galleries & musuems, pubs, sports, walks etc?
Amsterdam if you haven't been is really cool. Forget all the people who go there for sex and drugs - it has some amazing museums (Rijksmuseum, Stedjlik and the Van Gogh), you can cycle or walk everywhere, having a few beers in the Bruin Cafes (for the atmosphere) or Canal side pubs (location) is great, food is really good, and there's a heavy malaysian/eastern influence. Also, you could go for day trips to Utrecht, Haarlem etc. Lots of other things to see, such as the Anne Frank House but it's more a great place for relaxing and eating and drinking well. Plus, Vondelpark is great.
I'm probably going to take a week off in August as well and will probably either go to Aix-en-Provence (or else i'll go to Syria - small difference!). Aix is relatively small (more of a town), but has some really nice cafes and places to eat, and should be quite lively. Provencal countryside is amazing, and there are lots of other towns closeby - Marseille (vibrant, diverse, Boulabaisse), Arles (Roman Ruins, Van Gogh), Avignon (old city, probably will be thronged with tourists), or you have the Camargue, the largest Delta in Europe where you can ride the Camargue horses. Weather and good-looking ladies guaranteed!
Lake Como is great for a more relaxed type holiday, and you could combine with Milan for the more City-type things you like. The trio of towns (Varenna, Bellagio and Mennagio) are really nice, and when I was there a few years back there were plenty of activities (trekking, horse-riding, cray golf!)as well as the numerous villas and galleries. Main draw is eating on the lake every evening. More of a trip to do with a young lady.
Croatia good as well, and you could split your time between Split/Zagreb and Dubrovnik (stunning). Ferry from Split to Dubrovnik is great. Dubrovnik will keep you occupied for a week if you take some trips to the Islands. Not as cheap as you'd think though.
The best website for me regarding holidays (no matter where you go) is
www.homelidays.com . Thousands of apartments for rent, much nicer and infinitely cheaper than overpriced hotels. A lot work out not much above the cost of a hostel, especially if there are more than a few of you.