Re: Any tips for 'useful fidgeting'?
An alternative is to be in the moment rather than looking for distraction. That's the state you're in when you play.
Try really paying attention to your reality - the people around you, who all have stories - can you guess them? What about the design aesthetics of things around you? Contemporary? Retro? A particular period or style? How about the smell of the air? How about your breathing? The meditational in-the-moment stuff - noticing what your body is doing, where you are relaxed or tense etc?
I say this because I'm ADHD and ultra fidgety and I'm trying to train myself to be more mindful of the moment - more in touch with reality as it is right now, rather than past or future. I'm just a beginner doing this stuff, but I feel like this approach has potential for holistic improvements that will include drumming.
It's just one other option - nothing wrong with playing diddles or polyrhythms if it doesn't impinge on others in the queue.