Berlin has a very good public transport system, especially the U-bahn (Underground Rail), so it is possible to stay many different places in Berlin and still be able to reach the places you want to see easily and fast.
The main issue for you would be how easy or difficult it would be for you to reach the trade show.
For reference, the Check-point Charlie is at the Kochstrasse U-bahn station (U6).
A link to the U-bahn site :
http://www.bvg.de/en/Welcome

Public transport is fully integrated in Berlin so a ticket works for S-bahn, U-bahn, Trams and busses. The easiest is to buy a day ticket (AB - 6.70€ includes Tegel Airport).

I love the food court in the department store Ka-De-We :
http://en.wikipedia.org/wiki/Kaufhaus_des_Westens
6.+ 7. floor is food only.
Even if you don't eat there it's fun looking at.

Berlin has a lot of cheap or fairly priced sushi places.

There are many things to see in Berlin. Try looking at Wikitravel to see what interests you :
http://wikitravel.org/wiki/en/index....e_view_desktop

The busses 100 and 200 (doubledeckers mainly) are very good for a bit of sightseeing on the cheap. (normal tickets).
They leave from the Zoologisher Garten Station.