Frankfurt
Westend
Senckenberg Museum of Natural History (Naturmuseum Senckenberg) - One of the biggest natural history museums in Germany. Exhibits cover the evolution of the Earth, there's a great paleontology exhibit from Grube Messel, a pit with fossils dating back 50 million years.
Gruneburg Park - Large public park with botanical garden.
Playground (Spielplatz) - On Siesmayerstrasse, small children's playground, benches, picnic tables, grass to run around. Also on Siesmayerstrasse, restaurant - cafe.
Palmengarten - 50 acres of botanical gardens, here's lots to see, from a magnificent rose garden to an oriental rock garden, in the Tropicarium there's tropical vegetation.
In warm months, you can rent pedal boats on the pond.