Prague has a number of cemeteries that are used as quiets place to rest even when you are still alive. They are considered parks by the locals who go there for a peaceful walk (for example the Olšanské or Vinohradské cemeteries).
However, if you want to see something different, visit the Bohnice pet cemetery. It is not accepting new “customers” because it is full… And since the pet owners who had their pets buried here had a right to plant a tree next to the grave (and obviously the plots were very small), the place now looks like a jungle. But it is very interesting to look how deeply in love some pet owners were with their animal companions.
Across from this place is the old abandoned German cemetery which will give you an idea what happens to the cemetery when the people stop coming to take care of the graves for several decades. And again, if you are trying to escape the crowds, this may be the place to visit. But you should visit only during daylight, in the night it could be scary – there’s a reason they call this one of the scariest cemetery in Prague (satanic rituals were reported there in 2008).
The best way to get to this place is by bus (107, 160, 359) to station Libeňská and walk through the Psychiatric hospital Bohnice. (You will be able to tell your Czech friends that you were in Bohnice and hear them respond “Oh I am sorry!”)