Small 15mm combo wrench, since I have bolts instead of quick release axles.
2 hex keys, to fit every hex-head bolt on my bike
Small two-way reversible screwdriver (Phillips and flat)
Spare tube
Two (maybe 3) plastic tire levers
Tube patch kit (I like self adhesive patches)
Air pump, frame mounted
That's the essentials. I haven't found a multi-tool yet that has everything I need. But they all seem to have a lot of tools I don't need.
Throw in a fully charged cell phone and I'm set.