From what I've read, full body scans are awful for this--your body forms and kills tumors all the time. The false positive rate is ridiculous.

Ultrasound can also detect (some) kidney stones before they start moving and become painful, allowing an assessment of whether a medical intervention is useful or necessary. When I used to get kidney stones more frequently, there was a year or so when my doctor sent me for an ultrasound every few months to try to detect them in advance (!).

I think this is currently seen as too expensive to do for people who have lower risk, but I mention it as an example of something that one could check for more routinely given much cheaper ultrasound scans.

Prophylactic ultrasound exams are also apparently much more plausible on medical cost/benefit than prophylactic CT exams, because the CT exams very slightly increase one's cancer risk (https://xkcd.com/radiation/), where ultrasound doesn't.

(At a friend's doctor's suggestion, I started taking alkali citrate supplements and switched from almond milk to oat milk; I now apparently rarely get kidney stones.)