Like coffeechocolatewine, I have an aversion to tat but my DDs have rather large sacks so it's always fun to fill!
The bulk is made up with useful stuff: new clothes including tops, underwear, dressing gown if needed, tights etc etc. Any new stationery stuff that is needed or anything needed for their hobbies. This probably makes up 2/3 of their sacks and by and large is stuff they would get anyway, but all helps fill the sacks!
Then they get naughty treats that they're not normally allowed. Things like huge boxes of chocolately cereal (never buy at home - only have on holiday and at Christmas!), bubble gum and other favourite sweet treats. Atm, my DDs are into Oreos and we never buy them, so they'll each have a couple of packs in there.
A book / DVD / cd / game. Whatever they're into at the moment.
A couple of bits I may have seen and thought that they would like.
And three things they're allowed to request in their letters to Santa.
My DDs are 7 and 10 now, but we've always followed this formula.