Well, I suppose that would depend on what you don't already have. Is there anything that you need or would like to have that's small and relatively affordable? Do you have recovery gear, a CB, etc?
Lowe's has a real nice Extech auto multimeter with a little carrying case for about 27 bucks. It is great to take anywhere if you might be troubleshooting battery or electrical problems. It's a really nice reliable meter. This is great for anything you could need on the trail and it is really small!