I'm a fan of planes.
But for ground vehicles I would suggest getting simpler ones, and modifying the acceleration to be slower.
If you haven't picked a theme yet, I suggest planes, jetpacks, and hang gliders on a lava-pillar sort of map.
Like- have lava below and giant tall pillars rising up into the sky. The city-rpg takes place on top of the pillars. Players can travel from pillar to pillar via bridges, or they can use planes to get around.
Or you could do boats in a Venice style, streets of water map. Last time I tried that... it seemed fun. But the other admins just weren't interested in it.