Its a darned good idea to have a few essential tools with you whenever you head off on your bike.... just in case you need to do some on the ride tweaks or repairs. The easiest way to do this is to go for a bike multitool.
Multitools are cracking little devices which have all the essential tools you need in one handy gizmo. There are loads of alternative ones available from very small light ones with only a few vital tools, to big chunky ones with everything but the kitchen sink. We've got multi tools from brands like Pedros, Ice Toolz, Tacx and more.