My list, if I could remember it all would take many pages.
Some highlights:
Insulated and heat taped all of the plumbing lines, along with doing the electrical upgrades necessary to run it all. Insulated the entire underbelly, and added anotherl heater outlet to it. I should have running water down to 40 below.
Welded up an outside storage bin that fits between the frame rails and rear bumper. An angle iron and expanded metal affair about 5'.6" wide by 12" deep and 10" long. Great place for rugs, assorted lumber etc.
Installed a Winegard Traveler sattelite dish. I have HD TV at the push of a button. By the time I crack a beer, the sattelites have been found. It automatically stows too, lest I forget. Installed 1.5" lift blocks to allow for real LT tires to fit.
Welded up some custom battery security brackets.
Had a custom pillow top mattress made. (surprisingly affordable)
LED lights.
600 watt whole trailer pure sine wave inverter with an automatic transfer switch... All outlets are live all the time. Very conveinient.
Extra stabilizer jacks mounted in front of the tires to stop frame "bounce".
Upcoming project: Upgrade 5200# axles to 7000# axles, springs etc.
Have done many little things as well.
My previous two TTs I installed onboard generators. That really made them sell fast when it was time for them to go.