I teach fifth grade and, if you were looking for a book as a gift my kids have gone crazy over the "diary of a Wimpy Kid" books (I know someone else mentioned them also) and "City of Ember" (there is NOTHING inappropriate in it - a risk in some "big kid" books). If you do a book maybe some hot chocolate and popcorn for reading on a winter evening.
I know that the skate board things that have been mentioned are popular (I for get the name) and Bakugan (prob. spelled that wrong) balls. They look like a little ball and then they open up when placed on metal.
I think that gift cards are also great. THey do like having choices and feel very rch to go to the sotre with their own money.