Working in the industry I get asked a lot about gifts for DJs (especially at Christmas), so here’s my latest answer for you:
If you want something super-cheap then get him/her a USB disco-light. Absolute no-brainer for house-parties. Link here.
If you’ve got a bit more cash to splash a professional logo always goes down a treat. Link here.
If money is no object then just wrap up a Pioneer DJ controller. The latest one (Pioneer XDJ-XZ) is the biggest and best.