The Eliasson exhibit on Weather is pretty good. This year it features the sun. We really enjoyed it.

The building is fantastic, I'm still a bit neutral on the grounds.
If you are inclined, here is the menu for the restaurant.
Tate Modern Restaurant Menu
I would highly recommend combining the visit to the Tate with a short perambulation to the Borough Market for more wine purchasing, cheese tasting and what not.
Violist -- I think booking is essential on a Saturday, if that is the day you are planning to visit.
lala