Thunder's too early, Thunderbolt might work, Shock Wave too. How about a rock move? Rock Tomb? It's given out by Brock and Roxanne so it's not too early (like Thunder).
Or Sonicboom? :P It'd make for easy kills for leveling up (if you want to power level in the demo). Of course we could always change it to a sidequest AFTER the Demo, so people who go to him/her for the TM end up on a totally epic sidequest instead without any idea as to what they were getting into.