if you have 2 hours, here's what you do - get a taxi to drop you off in the old town square. if you can do it to corespond with the top of the hour, see the clock ring.
walk around the square, there is an interesting monument to the anti-catholic czech rebel leader.
then walk from there to winseslous square. at more or less the northwest corner of winseslous square, there is a side street that leads off to one of my favorite places in the world.http://www.pragueexperience.com/places.asp?PlaceID=649
if you stand in this spot, there are several very cool early 20th centrury buildings, a midevil building and a renasaince building in your view.
there are a number of small pubs type places to eat along this route, as well as suages places on winseslous square.http://www.pragueexperience.com/places.asp?PlaceID=1008
while urquell is fantastic, there are also some beers that are very good that you simply won't have heard of. in terms of what to eat - I would try czech style gulash (maybe they call it prague style gulash, but it is gulash with a piece of roast sausage in it) and bread dumplings. frankly, probrably not the best meal you willl ever have, but worth trying once.
if you have maybe 3 hours, maybe 4, do the same thing, but have the taxi drop you at the castle, then walk down and cross the river over the charles bridge, then continue.