If you want to visit one of my Areas, but I would highly, highly recommend going to the ward in Darmstadt. It's definitely the biggest ward I've been in on my Mission, possibly twice as big as the other two, and the members are wonderful -- very strong testimonies there, and a beautiful building. The ward Mission leader, Eckert Maichel (my-shil), is also an English Translator for work, so he's basically American (much, much better sounding than most Germans; flawless). I don't know a whole lot of sabbath appropriate things to do in Frankfurt, possibly visiting the Mission Office...besides that I don't know. But in Darmstadt there are a couple different parks you could visit, and there are some very very pretty dorfs in the area.
The Parks I know are Marienhöhe (Close to Alicehospital) with a very very Elaborate albeit small Russian chapel.
Also the Rosenhöhe (Close to the church building) actually has a lone Redwood tree (short one) and a beautiful expanse for a walk.
Luisenplatz has a historic Tower with a statue on top, that is probably the most recognizable Symbol of Darmtstadt.
Next to Luisenplatz, which is the City Center, there's another park called Herrn Garten that a lot of People go to, and off to the side of it is an actual garden with a church called Prinz Georg Garten that might be worth seeing.
Each of those places you probably wouldn't want to stay more than 15 minutes at each, so with travel I imagine it might take an hour to hour and a half if you don't go there for the Meetings. Also if you do get to go inside the Darmstadt building for some reason if you still go to Meetings in Frankfurt, the red carpeted "YSA Room" across from the kitchen, with a glass wall, was the room where I'd do district Meetings while I was there as district leader.
I love you,