Most of the better things I compose in my head fall into three categories: string quartets, chamber music for larger groups (6-10), and short concertos where I can just play with the sonorities of the soloist vs. accompanists, knowing I'll probably forget most of the material in a few days anyway.
Some pieces I think about quite frequently are a fantasy on a Turkish folk song, modeled after RVW's Tallis fantasia but for string quartet; a jokey divertimento for string quintet + wind quintet; and chamber concertos for instruments like oboe or trombone.
This has changed quite a lot over time. In middle school I couldn't remember a single made-up melody from one minute to the next, so while waiting for the school bus I'd mentally "play" improvised Mozarty piano concertos with absolutely no structure whatsoever.

Like, it would go, "here's a happy intro melody, here's another happy melody, piano enters after a few minutes and...plays a new melody, and then they all play yet another new melody, cadenza, and maybe the first movement ends with some sort of half-remembered, vaguely similar callback to one of the previous ideas." Total disaster, but hey, the point was just to kill time!
And then in college, there were a lot of moody romantic tone poems, which is a form that can sometimes have a total lack of structure in real life. Sometimes now I'll hear a moody romantic tone poem on CPO or some other label where it's 20 minutes of gloomy melody-free dithering around, and think, "ugh, this sounds like something I made up," and turn it off.
