I thought of these this weekend that I didn't carry with my all last season, but will starting on my next outing.
For the Trailer:
Lug wrench and jack for the boat in case of flat.
Spare tire for the boat (I still can't believe this doesn't come with the trailer)
Spare locking pin for the tongue (drove off without the lock pinned down twice now)
Boat:
Spare key in the dry box
Small tool kit (basically a leathermans worth of tools, and a spark plug socket)
Pocket knife (Wal-Mart sells a $1 knife that is sharp for a few uses, I keep several on board, if it gets dropped, no big deal)
Stash of Micro-Fber Towels
USB/12V powered fan.....Had a couple feel "heat exhausted" while on the boat. Soak a towel in water, lay them in the shade, and throw a fan on them.
100 ft of generic 3/8in line......No idea when you need to tie something to something else.
Big Clear Tarp/Drop Cloth and some spring clamps........This sounds wierd, but if a storm/shower blows up I can toss it over the bimini, clamp it around the edge of the boat with some spring clips, and keep MOST of the people and stuff inside dry. It looks absolutely ridiculous, but was cheaper than a camper canvas, and works. We got SOAKED on Memorial Day last year without this, and have weathered a few storms/squalls/showers since with it. You can't get on plane with it, but you can motor around over idle speed with it and still remain dry. The clear-ness of it keeps everyone from feeling claustrophobic, and aids in navigation while attempting to return to dock (or any other shelter you can find near the water to let it pass).