What to do for 2 days in Frankfurt in January?

Here's some less obvious possible day trips. Seligenstadt is very similar to the better known Rotheburg, with a large abbey complex (no town wall, though). Aschauffenburg likewise has the same types of fachwerk buildings, and also has an impressive Schloss in the middle. I also like nearby Wiesbaden. Its very elegant, but kind of laid-back.

You might enjoy a train ride along the scenic Middle Rhine Valley, northwest of Frankfurt. Travel to Wiesbaden and then north along the east bank as far as St. Goarshausen. You'll spot many castles along the way. Take the ferry across to St. Goar, an attractive village, visit Rheinfels Castle (open weekends 11-5 weather permitting) and see the town, then catch a train south along the west side of the river, doing some more castle-spotting from a different perspective, and stopping in Bacharach, one of the more attractive villages, if you have time. There are some inviting places in Bacharach and St. Goar for a meal. Mainz, which you will need to pass through on your way back to Frankfurt, also makes for an interesting stop; the Marktplatz (market square) and the Dom (cathedral) are adjacent and a good walking destination from the station if you want to explore the old town area (just to the south of the Marktplatz.) Trains from Frankfurt to St. Goarshausen are direct and take about 1.5 hours. It's a little longer returning from St. Goar. Figure about 36 Euros for the train journey + 2 Euros or so to use the ferry. Get your schedules here: http://reiseauskunft.bahn.de/bin/query.exe/en