The best mod hands down we inherited from our wonderful previous owner: an aft cabin conversion into a single wall to wall bed taking normal mattresses and bigger than king size bed. For a three person marriage, it just about sold the boat for us.
Another brilliancy was to put a waeco freezer in place of the nav station stool...with a solid enclosure and a cushion on top, it rocks.
We cut out the admittedly beautiful table leg wine holder, and replaced it with two robust pistons to convert it into a huge daybed/guest bed/movie couch, with a 40 inch screen opposite. Also wonderful. We usually eat in the cockpit, or on our laps, so we have raised it maybe twice in 3 years. We enjoy our movies so that mod was pretty necessary, sitting around the table for film nights got old in about 2 weeks. All the bilge space was preserved underneath, and the wines are in a milk crate under one side of the seating.
A tiny mod I did immediately which has been replicated at least once for being so handy: I cut out a 3/4 inch half circle from the top of the bin holding the shower hose...that old hose is precious, and now it can be out without getting squashed by the lid. There is no need for that to be 1000 percent watertight as the SM cockpit stays dry.
Edit: another ESSENTIAL to us mod our po made: two additional hatches in the roof of the aft cabin. I don't see how people survive summer without it. One over the head, the other in line with it over the entrance. We pull the back of the dodger out over the sundeck with a line in the rain, and can keep these open under almost monsoon rain. Gets the steam out of the shower in 30 seconds too, and they face forward to scoop air on anchor. Absolute must for us...
We installed a litter box under the galley sink, but I don't think anyone here is interested in that 😅