stemming from my own experiences having a roommate you hold a healthy friendship with, i think dennis and trinity buy each other terrible gifts relating to the things they experience.
for christmas, dennis gets an ugly avocado cross-body bag to put his work badge in, so he doesn't lose it again. trinity gets one of those stupid t-shirts that says "this is the only shirt i had with no cum on it", based on a horrifying discarded laundry item he witnessed crossing into her room once. after a truly terrible shift that is heavily discussed, one of them gets a plastic "employee of the month" trophy. (that then goes on display on the mantle or the coffee table).
much to trinity's dismay, after long enough living together, she realizes they've also begun matching in many of the things they have. mugs, pajama pants with cartoon characters, candle scent preferences. they don't have to ask questions so much anymore when shopping.