Hello folks!
This is a much delayed post. I had it in edit mode so long that I forgot about it! But better late than never.
Anyhoo, on your next Frankfurt layover, be sure to stop by the Palmengarten. It is a very short 13 minute (0.7 mi) walk from the Marriot Hotel.
Directions (Google Map):
Go north on Senckenburganlage, and a right at the partially submerged trolly car (subway entrance, really) onto Bockenheimer Landstrasse. Take a left onto Palmengartenstrasse (it's called Beethoven Strasse on the right) and there you are!
The cost is 5 euros, and you can spend as long as you want in there. Check the concert schedule as there may be a concert that you can see also. There are a couple of cafes on the premises, so you can grab a cup of joe or a bratwurst and do some naturewatching.
There is a mini-railroad for the kids as well as a small splash water park. The train wasn't running during my visit, and the splash park I'm sure is seasonal.
If you're a jogger, you can waltz over to the Gruneburg Park across Siesmayerstrasse for some nice running atmosphere...
Don't forget your camera! Enjoy!