Apart from Taj what we visited was the Old fort or Agra Fort. A huge red coloured fort with gardens, water springs in it that looks exactly the way we see in Movies and TV shows. Upon reaching we were welcomed by a big door that once used elephants for protection. The whole fort is big, big enough for you to spend and enjoy 3-4 hours.
After these visits you can stay overnight in Agra. The next morning you can visit the market and streets of Agra if you like. OR you can straight away go back OR you can stay for a few days and can visit other places near by. Its all how you like things to happen.