I have not yet been to Prague, but many friends have visited it and tell me one does not need a car there. In fact, like many European cities, having a car during is actually more trouble than its worth due to lots of traffic and limited, often expensive parking . Between walking, public transit, and Uber, I never rent a car when I visit any major cities in that part of the world.
I recommend you start picking out a few places to see via TripAdvisor, and Flickr, etc, so you can make a rough plan of places near your hotel to take in after the workday is done, and plan some longer trips if you have a free day.
One thing you won't lack is subjects to photograph, Prague is a large and popular city, so there will be all sorts of activity in the streets all the time, especially along the river.
Others have given you some great ideas, I'd like to recommend the
Alphonse Mucha Museum in Prague. Mucha's illustrations influenced many other artists, and his series devoted to the history of the Czech people is worth seeing in person, Plus, its a good idea to have a few indoor locations in mind if the weather turns bad.
I look forward to seeing your pictures.