Chain tool: if you do this infrequently, buy one that can go into the bike bag.
Metric Allen Wrench set: Obvious
Cassette removal tool: You are going to do this eventually.
Cable cutter tool: Learn to do your own cables and you will learn how to tune bikes.
Spoke Wrench: get the color that works with your bike...........black for DT and Wheelsmith spokes, red for japanese spokes. If you have a choice, get a 4 sided wrench as you will round off fewer nipples while learning.

This will cover most of the work a new mechanic will want to take on. As you learn to do more, there will be more tools to get.