The other thread is great, but I think we need one for strictly HTT mods (even though some of these could be done on any trailer), since some people might not frequent the TT forums.
Here's a couple of mods we did over the last few months.
Electric Tongue Jack.
Digital Thermostat:
Rails and ladder for the bunkbeds:
Don't know if this one counts as a "mod" or not, but we also installed a 30 amp service for the camper:
Installed Pop Up Gizmos and Reflectix inserts under the bunks and in the windows of the bunks:
Got some Bal Tire Locking Chocks to eliminate the "wiggle":
Got a few more ideas floating around for some mods this off-season, too.