Hi Dev,
The outdoor pool season has started late here in Frankfurt because the weather has been so bad, but most of them are open now.
You´re pretty much guaranteed of getting some space to yourself as the locals tend not to go swimming unless the weather is 85°+, which at the moment it definitely isn´t!
Keep an eye on the thunderstorms which we´re getting on a daily basis, the lifeguards will pull you out if there´s any risk, which can be a nuisance.
Anyway, here are a few possibilities:
Waldschwimmbad Neu Isenburg: 50m pool, heated, not busy. A really lovely pool.
http://www.waldschwimmbad-neu-isenburg.de/de/Freibad/Oeffnungszeiten-Preise/Oeffnungszeiten-Preise.html
Bad Vilbel: nice town, just near me, the Freibad is apparently open although I haven´t been yet. Empty during the day, gets busy at weekends. 08:00-20:00, Huizener Straße, Bad Vilbel.
Stadium Freibad (Stadion) - currently only open from 07.00-10.00 (Frühschwimmen). VERY convenient for an airport hotel. 50m pool. Bearing in mind your schedule this is probably the one to go for.
Brentanobad - currently closed but this is definitely the one. Swimmable length of 125 metres!
Hausen - next door to Brentanobad. 25m heated outdoor pool, very nice.
I would strongly advise against getting a hire car. I´m happy to drop a road bike off to your hotel (with a decent lock and lights!) for you to use during your time here. Probably a 59cm. Plenty of nice roads to be discovered. That way you can zip about the city etc etc.
Cheers
Simon
Simon, this is why ST rocks…but I fear a 59 cm bike would be the length of my inseam (OK, I am not that bad…but it fit best on a 50-52 cm bike)!!!