Having been in Berlin for a long weekend last week I have a fair bit to share!
Hotels are everywhere and all reasonable prices really, we stayed in the Hilton in Mitte which was nice but business bias but we got a good deal on expedia so cant complain. there are others available and anywhere around Unten der Linden will be well placed and lively. The Oranienstrasse area was good for bars and resturants, german cusine is utterly awful based around pizza, pasta and sausage... the beer is awesome though.
the u-bahn/s-bahn tickets are really cheap, a 7 day zone A/B/C card was 33 euros, you will need at least zone A/B. its a big city and the u-bahn is very reliable. its all very well signposted and there are maps on the platforms.
things to do.
The East Side gallery is pretty unique, something like a mile of teh wall still intact and loads of murals etc. there is cycle shop/cafe called keirin http://www.keirinberlin.de not far from there too!....
huge shopping centre in alexanderplatz if its wet
The soviet war memorial in treptower park is amazing and worth a visit.
the concentration camp in sachenhausen is also worth visiting, yes its humbling, terrifying and eye opening. its on the s-bahn (zone c) about 35 minutes out of town. its always open but the indoor exhibitions are shut on mondays.
the mauer park flea market on sundays is pretty good fun, loads of stalls in the market and some musicians/street performers in the park itself.
tiergarten is a big park, worth walking through if its nice. loads of bike hire places around too, worth taking advantage of. about 10 euros for the day.
the holocaust muemorial just south of brandenburg gate was weird, the memorial itself is quitebleak and the exhibition underneath is an eye opener too. you learn all this sort of thing at school but being there really hits it home.
the absolute must thing to do is the Stasi museum, http://www.stasimuseum.de/en/enindex.htm
the acutla building where the SS operated has been kept as original, you can see all the spy equipment and the Head guys office is as it was, its an amazing place. it just looks like a 50s office building but inside has all these reminders of the stasi activities. definately the best thing there.
checkpoint charlie, well go there, but dont hang around and the museum next to it is very cramped and quite pricey (12 euros each) its worth walking there from the top of Freidrichstrasse down.
loads of good cafes, Cafe Einstein was one of the better ones though they sold out of croissants by 0930!
tahts all in the old East side, the west side, as mentioned by others, was nice and worth wandering through. In general, it was nice and safe, friendly and a typical lively central european town.