Hmmm, when I was pregnant with DD towards the end I was being monitored every 3 days and one of the things they wanted was to check her movements - so here's all the things we tried:

Sugar-rush - drink a can of lemonade (or coke if you want to be naughty) about a half hr to an hr before going in for the scan

Cold stuff - icy poles work a treat as they combine cold with sugar to get them wriggling.

Poking wars - I'd find sometimes if I poked my belly a few times DD would poke back or start wriggling around (though this could be more a very late preg thing)

Loud noise - turn the radio in the car up and sing along on the way there. Plus it's fun

Good luck!