We deal with children in all grades, which enables a volunteer to pick what age group he/ she prefers. So, if you are with the younger kids you may help them learn how to add 9 and 2 or spell a few words, whilst if you are with the older kids it gets a bit more hairy, like deriving the equation of a parabola or helping with some history or accounts.
Other than that, we do things like reading for the smaller children, taking the kids out for their birthdays, buying them presents and some other activities as they arise.