The Christmas market (in and near the Marienplatz) will still be operating on the 24th from 10 AM to 2 PM and is well worth a visit. Things will be largely back to normal on the 27th and 28th. For the 25th and 26th, the best recommendation I can give is to visit churches. If that appeals to you, the Frauenkirche (near the Marienplatz), the Asamkirche (on the Sendlingerstr.), and the cathedral in Freising (about 25 miles north, the last station on the S-1 line) are all quite attractive.