You are staying in Prague. You've spent a few days looking at some of the best architecture and art on the planet, but you are getting a bit tired of city streets. You long for fresh air, open spaces, and silence. Or maybe you're a runner, or a walker, or a nature enthusiast, and the cityscape is beginning to close in on you. Well, you are in luck.
Kuntraticky Les is at least 3 square km of forest, and it's only 15 minutes from Old Town Square. I have listed the Roztyly Metro as its location, because this is easy to get to from the centre, but the Forest extends much further. A taxi can take you to many other access points. Just be sure to work out how to get a cab back again.
There are wide and accessible trails all over the forest, suitable for walking, biking, and if you are up for adventure, even wheelchairs.(caution: these are forest paths, not tarmac.)
There are at least three restaurants serving local Czech food and delicious beer at local prices, with play areas for kids. There are streams, cliffs, trails, dozens of kinds of trees, wild flowers, birds, deer, hares, rabbits, and a flock of wild Mouflon sheep. If you see the sheep though you are lucky, as they are easily scared and try to avoid people. There is even the ruin of a genuine castle which has no access restrictions whatsoever.
If you enjoy a walk, the outer perimeter track is about 12km long, but there are many cut-throughs and short cuts. Just use Google maps, or even better buy a paper one before you go there.
Summer, Winter, Autumn, Spring. There's always something to see, and the tranquillity is amazing.