Geez, just when I thought I might be about done spending $$ on the new TT, I see all these great ideas!
So our first upgrade was getting a real mattress instead of the blanket on a board that came with the trailer. DW picked out a queen size 10" memory foam mattress and I can tell you from our maiden voyage it is one of the best RV upgrades we have ever made (and we've made a lot!).
There was a bit of a consequence to this though - as you are probably aware a trailer queen size is 6" or so shorter than a real queen size. So I built a little shelf to support that overhang. My intention was to use it for the first trip and then look for some sort of shelving unit to put under there like mchayes did. However, our wonderful fur balls had a different idea and they like it so much we will just go with this being their place (better than under foot). This means I need to go back and stain & varnish the wood so it matches the TT wood but that will be easy enough.
The other down side is that the mattress weighs 90# so lifting the bed is a real pain. That means I won't be partitioning off the pass through like others have. I'm still glad we did it - I sleep great in the trailer now and that's worth a lot!