It will depend on what they like to do. If they like cars maybe a ticket to the V8,s, if they like body boarding it may be a set of fins, cricket it maybe a ticket to a world cup match or if fishing it would be fishing tackle or a fishing charter.
I'm a big fan of educational toys. Age-appropriate books are great, as are science kits and games, which are available in many forms and in various price-ranges. Gift shops at larger museums are always good places to find high-quality children's books and toys.