Rome

This place is so full of amazing things, you'll be a drooling braindead monkey by the time you leave. As the guy who owned our hostel told us, don't spend more than 3 hours in the Vatican Museums or St. Peter's Basilica - any more than that, and your brain just can't process it anymore. Rome is that overwhelming. It is full of all the famous things you read about, but once you are here, you will find something around every corner that will blow you away. As I usually recommend, plan to see a few big things, and the rest of the time - just get lost. Wander around, I promise that you will not be disappointed with what you find. However, you really shouldn't miss the Vatican Museums - just make sure you're in line at least an hour before the doors open, otherwise you'll wait forever and will encounter ridiculous crowds inside - the same goes for the Coliseum. I also recommend exploring the different piazzas around town, just spending time there chilling out and doing whatever the locals are doing. We were here for four days, and must have walked tens of miles every day - still we barely scratched the surface of things to see in Rome.