If you are just looking for something to do, perhaps with a few friends, may I recommend taking the afternoon canal cruise?
The Stern und Keisschiffahrt company offers a 3-hour cruise along tge Landswehr canal through various neighborhoods, past many points of interest which you can explore later; it reaches to the west of Berlin center, and then returns along the river Spree, right though the heart ofthr city and past the Bundestag and the chancellor's residence, and past the Berlin Cathedral and Museum Island; past a few embassies, along the remaining portions of the Berlin Wall, past the Mecule Men sculpture, under a charming bridge, and then through a set of locks to bring you back to the level of the Landswehr canal, finishing back at the Estrel biergarten/strand.
The tour has a guide, who can give the tour in german or english (when I did this 3 years ago the passengers were mostly german vacationers - perhaps 20 or so, staying at the Estrel - and I was the only non-german speaker in the group. The Guide handed me the binder with photos of each site and bridge that we passed by or under, along with the commentary in english to match what he was telling the others in german. Each bridge has a name, written underneath (and almost every bridge has a story!); I had no trouble following along.
The advantages of this tour are:
1). Its on the water - it is cooler than walking along the land, especially as you are moving, and if a breeze comes down the river or canal, even cooler.
2). EDIT - removed the mention of drink service, because I have taken 2 other cruises by this company which had that service, but I don't actually remember if the canal cruise did; PLEASE ASK THE ESTREL CONCIERGE about this point.
3). You actually get a useful tour of the city, and can marvel how much of it is accessable by water.
4). Its RIGH THERE! You dont have to figure public tranport to get to or from it, the boat ties up each night right there at the Biergarten. Tickets are avalable from the concierge in the hotel, and if a roomful of you go, he can probably charge the tickets to your room, so no outlay for cash.
5). It is a pleasant way to pass the time, either alone or with close friends; aside from the need for each of you to buy a ticket, even a large group - say, oh, 20 or so - can go and sit together.
Please excuse the spelling - I'm typing this on an iPhone and my vision isn't what it used to be.