So my 3-way divided 10-gallon tank wasn't working out. Personality conflicts, etc. I've now got my boys in individual 2 or 2.5-gallon containers spread throughout the high-traffic areas in my apartment. I've decided to make the 10-gallon into a community tank with another betta and a small shoal of Harlequin Rasboras. I've added 3 rasboras already--I'm waiting for a week or so to add a few more to the tank. They're really active fish, and their new game is to swim back and forth through the filter baffle. They're a blast to watch. Gary, whose tank I've placed next to the 10-gallon, seems to think so too.
I catch him like this all the time. Never flaring or anything. Just watching them swim up and down along the glass. It's like the fish version of Wipeout.
Gary's other favorite thing about the new tank, for whatever reason, is the thermometer. After he's tired of watching the rasboras he'll often swim over to the thermometer, stare at it for a while, maybe nip at the suction cup. He guards it fiercely anytime he detects an intruder in the tank (i.e. the turkey baster I use during water changes). I'm not sure why he's developed such an attachment to a piece of plastic, but it's really amusing.
And yes, there's usually a lid on his tank--I just took it off for a few minutes because I can't get good pictures through the side (the tank's a cylinder). Gary discovered that he could jump last Friday while I was doing my 100% water changes. He didn't make it out of his cup, but I won't be taking any chances now.