Multi tools
Crank,
Here are a couple of ideas. The most bang for your buck!
Either the Topeak Alien XS. It is a 17 function multi tool. Or the Topeak Alien it has 22 functions. It includes a Chain riviot tool along with some others that the XS does not include. The reason I like the Topeak brand is that each tool extends out to be used individually. They are not all jammed up into one little tool rendering them virtually useless. The also come with their own handy dandy carring case. You can mount it to your bike with velcro (velcro is not included) or carry it on your belt.