Thanks everyone for the suggestions on how to get Radar to settle down and return to the urn, but I inadvertently found a way. It was quite fun watching his antics, and I would have left him alone if he didn’t keep waking people up at night. He really enjoyed going down to the Immortal Gallery and studying the paintings and photos. I think all the ice sculptures and furniture made him feel right at home - he wasn’t the only one who was transparent.
Radar was a bookworm when alive, and I presume ghosts keep their traits. Here he is reading over Millie’s shoulder as she works on her next Masterpiece novel.
After several days of him drifting around looking for something to do, I decided to let him go to the park with Millie and the baby.
Unfortunately, when it came time to go home, I couldn’t figure out how to get him to come with the others. We left him hanging around the park.
He stayed there until it was quite late. I don’t know exactly when he came home, but when the others went to bed I checked his urn and he was in it. Now he and his urn are in Mojo’s inventory where they’ll stay until I think of a better idea.