I took the regional train to Mainz yesterday, which is a journey of about 45 minutes from Frankfurt. Once there, I tried and failed to find the Tourist Information office. No matter; I found my way around by looking at bus and tram-stop maps.
A different river. Not the Main, but the Rhein. As you can see, a fake beach had been created on the riverbank -- complete with volleyball court, deckchairs and (as you can't see) bars.
Mainz has some nice cobbled streets, lots more quaint old buildings and some interesting shops. Like in Frankfurt, there's quite a Turkish influence. No end of kebab restaurants. I guess I spent about six hours in Mainz before taking the train back to Frankfurt. The fare, by the way, was 7.15 Euro each way. I had considered going to Heidelberg, but that would have been more than three times the price.
Back in Frankfurt, I followed a recommendation from one of the Antequera wedding guests and sought out Pizzaria 7 Bello (Niddastr - near the Hauptbahnhof). I sat outside, actually in the road rather than on the pavement, and enjoyed a Hawaiian pizza and a cold beer. An ideal way to spend part of the evening with the heat of the sun withdrawing to a very pleasant level.