Red Balloon is awesome. As are vouchers for a nice restaurant or similar for the in-laws.

Skateboards are way more than $30 for the 13yo, but you can possibly pick up a bunch of awesome accessories if he is already into skating etc by walking into a skate shop and talking to the people there. Otherwise there's things like movie vouchers, itunes vouchers, a good belt etc. Depends on what he's into really, so could be worth talking to his mum directly.