What to do in Prague
Do not have a meal outdoors in the Astronomical square. You will be stitched if you do. (A lot Dearer then London in a cheap city)
Do go to the miniature Eiffel tower above the town. Take the Funicular railway. Do not pay to go up the tower. Go instead to the free exhibition in the basement. Be sure to take a camera, and be prepared to be sick laughing.
Concerts: There are around 8 concerts a day held in the old churches. Get a flier, and just go. Inexpensive. Each concert lasts around an hour. Churches are beautiful, and you will get to hear some brilliant professionals playing everything from Mozart to Bach, with lots in between. I normally manage 2 per day.
There are 2 opera houses in the city, and I usually squeeze in at least one opera per trip. Last time it was Don Giovanni. Bit dark for my tastes, but brilliantly performed.
Places to visit. Theresienstadt concentration camp is around 60km from Prague. Well worth a pilgrimage. Be sure to get a guide.
Do not take a boat trip. They are horrible.
Jewish tour of the Old town.
Be sure to visit the Pinko & Spanish Synagogues.
The Pinko is now a most moving memorial to the Holocaust, and, whilst it portrays none of the dreadful things which happened, it is also so sad it is beyond words.
The Spanish synagogue is beautiful beyond words.
If you do decide to visit the Synagogues, go early in the day, and not on a Friday. the ticket seller for the tour is surly and rude, just ignore it.
I agree with everything else already posted.
Regards Myke