I prefer proper allen keys to fold up ones.
A leatherman tool with pliers is always useful. I use the small mini version.
If you want to bring and adjustable spanner/wrench then make it a good one. Swedish-made Barcho ones have minimal play in the adjustable bit.
Take the splined tools you need to removing cogs and bottom brackets. Even if you can't use them, a bike shop can.
Using a threadless headset means that you dont need to carry two large thin headset wrenches.
I usually carry some chain lube but if you dont you can always retrieve enough oil from a discarded can at a garage.