I’m in the same boat as I bought my first travel trailer last fall. The main upgrade I did was change out the four stabilizing/leveling jacks. The ones that came on my trailer were bent and looked way undersized. I bought four Camco EAZ-Lift 24-inch Heavy-Duty Scissor Jacks with 7,500 lb Load Capacity (Amazon). While not recommended, these jacks will lift the trailer off the ground without bending. The other upgrade I did was adding a sliding blackout shade to the overhead air vent in the bedroom. I didn’t like waking up to the bright morning light that the factory vent let in. Other good things to have are: a tool set that meets the needs of your trailer, good grill, stuff for starting camp fires, TV add ons to meet your needs (ie. streaming device), chocks and blocks, weight distribution hitch, small bungees to keep cabinets closed while under way. Not a full list, but hopefully this will help.
Keith