Tires

As long as the tire size load and speed ratings are correct you can put whatever tire you like on. Just match the tire to your driving habits and weather conditions. Generally for "normal" use an all season tread will be good for most driving except for deep snow/ice.
A site like Tire Rack can show you multiple tires rated to fit your van and has reviews of some. Keep in mind that those four small patches of rubber are what ensures your safety.