The best thing I did in Prague was take this boat cruise. There are people dressed as sailors who stand at all places throughout the city selling these boat cruises.
We got ours from the end of Charles bridge in the old town and for just CZK 290 (approx £6) we got an hour tour around the city with a guide who spoke excellent English as well as other languages. We were given food, ice -cream and a choice of drink (beer, wine or soft).
Fantastic value for money.
This would be good to do when you first get to prague as the guide tells you a lot about places around the city and it helped us too decide where to visit next.
You also get vouchers with your ticket for discount at the black light theatre in Prague.
Dont forget to take your camera as you can get some fantastic pictures of the beautiful sites around Prague.