You can make a reservation to tour City Hall (and the Tweed Courthouse behind it) on any weekday. Actually, Wednesday tours don't require reservations. Just check in at the NYC & Co. booth on Broadway just south of the building. The tour starts at 11, but you'll probably want to come earlier in season, as spots are limited to 20, I believe.
The building is more beautiful inside than it is outside. The dome and the grand portrait room are particularly impressive. The city council chamber is less attractive. It originally served as a couple courts and was remodeled with an awkward gallery that wrecks the proportions of the room.
My tour guide was less than ideal. She routinely took five minutes to say stuff that could have been said in one and she lacked energy. Ah well, I did get to see Mayor Bloomberg. He walked over and said "hello" to the group.