They are largely in Okinawa. As a resident of Japan, from my observations:
Assaulting the locals, murdering the locals, sexually assaulting the locals, getting very loudly drunk and doing one of the previous, getting very drunk and driving in local towns, dropping things from helicopters on local schools.

They also do joint drills with the Japanese Self-Defense Force, but a lot of those drills are not functionally very useful given Article 9 of our constitution. Technically we changed our position towards collective self-defense (to allow it) a few years back, but it has never come up in an actual engagement.

They did some assistance during the Fukushima disaster to be fair (about 20% of the troops stationed), but the vast majority didn't. Many of them were focused on evacuating their own families stationed in Japan, which accounts for another 7000 people not included in that 50,000 number. Those that did assist explicitly did not go anywhere near the areas that needed the most help (like Fukushima itself).

"145 members of special units called the Chemical Biological Incident Response Force (CBIRF) were dispatched to Japan, but their activities were limited to outside the 80-km radius from the Fukushima nuclear power plant; they went back home after staying for three weeks without doing anything other than waiting at the far area from the nuclear power plant."

Short Answer: Nothing Useful.

They are developing new advancements in the fields of racism and misogyny.

That's where we get our used women's underwear.

The 50,000 we have here is actually all US troops, nothing UN-related about them. We just are that heavily occupied. It is consistently the single biggest topic for Okinawan politics because of all the problems they cause locally.