a small selection of nuts bolts and washers.
carrying them with you at all times will ensure that you never actually need them.
The second you don't have them will be the time you need them.
seriously though...
A decent selection of screwdrivers (flat and cross blade), avoid the cheaper ones that will fall apart (Stanley has always been my go to make)
small spanner set, the type with a ring one end and open the other (you can tie string through the ring to save losing them in the cut if needed)
small socket set
2 sets of allen keys (imperial and metric)
some hose
some jubilee clips
tie wraps (6" and 12")
Not sure what the previous owner of my boat was up to, their toolkit consisted of 1 bent screwdriver, 1 pair of molegrips, 1 saw, 1 hammer and a spirit level