This company:
http://www.neweuropetours.eu/ offers free walking tours in many of those European cities. I only had 1-2 days in each city when I was over there and they definitely cover most of what there is to see in the cities. All the tour guides I was with were young 20's too, all very enthusiastic and full of energy. I'd highly recommend these...the pub crawls that they put on are fun too if thats your thing.