I think I'd try to maximize my time and take the 2 hour walking tour first thing in the morning then maybe a museum and/or Spree River cruise in the afternoon.
My favorite museums are the Egyptian, Gemäldegalerie and the Pergamum. The museums on the island are still undergoing a major renovation--check websites.
I haven't been inside the TV tower so I don't know how long it takes to see it. It's a presence you see from every view in the city naturally.
If you do go to Potsdam just be aware that many business close at 5 pm even in the summer. When we went to Potsdam last summer we discovered a couple of things. There is a slow way and a much faster way to get to and from Potsdam for maximizing your time, pay attention to transportation options. Also it was 100 degrees when we were there in August so we took a lake cruise on a steam boat in Potsdam. The San Souci Palace is one of my all time favorites in Europe but we didn't visit on this trip. We explored the gardens by ourselves in the heat instead as the previous time we were there it was in the middle of the winter and the gardens were barren.
If you don't go to Potsdam, you might go to the Reichstag dome if that interest you and you book tickets ahead of time.
If you don't go to Potsdam and you see something that interests you from your walking tour, go back and spend more time there.