It won't be raining in the hundreds of bars mate
.
Seriously though, been going to Prague since 2002 and been 10 times, recommendations would include...
Try to avoid British/Irish bars and bars around Old Town Square & Wenceslas Square. There are much better and cheaper bars elsewhere and there's plenty of them.
Get a 3 day travel pass, available from more central Metro stations and the Info booth on Wenceslas at approx £10. It's valid for the Metro, Trams and Buses, but make sure you stamp it in the machine before your first journey, think it's a £40 fine if you don't.
Charles Bridge, Old Town Square, Wenceslas Square, Castle area are all standards, but also try to go to...
Vysehrad, the other castle,
Petrin Tower, local version of the Eiffel Tower and go to it on the funicular train from Ujezd,
Stravov Monastery and Brewery,
Zizkov TV Tower, sputnik style tower, great views on a clear day,
Vitkov Memorial, massive equine statue and viewing gallery,
Municipal House, grand art deco building,
River cruise, there are night ones with a band, buffet and bar,
Plenty of churches, for the architecture alone if you're not religious,
Josefov, old Jewish Quarter.
Sure you'll find lots to do and enjoy your trip.