Bit stuck for my 13 and 12 year olds - was the same last year and finally ended up with a PSvita for the younger one and a drawing board/art stuff/graphic tablet for the older one.
They already have a load of techie stuff (I know they are spoiled) so there is very little along that line that they need. No2 son could probably do with a new PC or laptop but that would be a bit over budget plus he doesn't really use it much, usually plays on PS3. We have large family that generally gives them cash - hence they have saved and bought most of the bigger stuff but it also means that as they get cash from everyone else, we can't really do the same yet as they are still young enough to want to wake up to Christmas presents.
Not into sport or music, have bikes and scooters and have already done a lot of the gift experience type things that are near enough for us to get to.