If they're in any way into writing, get them a few postcards and some stamped envelopes, maybe a couple of stickers and a cute pen or two and make up a plan of writing each other letters. I'm penpals with all of my old kids (from ages 5 to 16 now) and I know they love receiving letters addressed just to them.
I also always take them out for a special outing on or right before the last day, usually for an ice-cream and a chat, or something the kid really likes. One of the boys was really into boats, so we just went and watched the river for an hour as his special parting gift.
You know what they like, you could get them each a book about their hobby with a nice inscription. I always go for books geared at a bit older age, so that they have something to grow into.
Good luck! And let us know what you got them in the end. There's loads of posts with questions like this and then you never find out what the OP did in the end, and I'm always so curious!