First thing I’d recommend is installing a master kill switch that shuts off all power. This saves your battery from power-robbing electronics and is an important safety feature.
If your boat doesn’t have lighter plugs for phones and other accessories, add them now.
If you don’t already have it, buy a lug wrench that you know fits the trailer lugs. There is about a 99.975% chance your trailer lugs will be different from your car lugs. Obviously, if your trailer doesn’t have a spare tire, now’s the time to add the tire and a carrier mount, with lock.
If your trailer didn’t come with LED lights, save yourself the pain and misery and install them now. Also install the walkway plates, either plastic or steel.
As long as we’re on the subject of LEDs, if your boat anchor and nav light didn’t come with LED bulbs, replace them now.
Internal lights are a great addition. Red is popular not because of personal color preference, but because it does not kill your night vision. Very important when on the water.
Also, buy a good LED spotlight that has its own batter, but can also be plugged in.
Grouse