Stromovka is a large park near Výstaviště Holešovice - an exhibition center, and a planetarium.
It's great for taking a walk, running, riding a bicycle.
There are wide grass fields, suitable for sunbathing or having lunch. But there are also dogs around, so, I'd guess it's not entirely safe to step on the grass, like anyone cares anyway :D
What the park misses is public restrooms, haven't seen even portable ones, and drinking water fountains in hot summer weather.

Beautiful park especially during blooming period in spring or in summer. Nice for walking around, biking, skating or running etc. Also there is a special area for grilling, but you should bring your grill with you.
On the territory of the park you can find Planetarium and playground for kids next to the restaurant/cafe.