Launching Grunts and Jackals
You can have some excellent sport launching Grunts and Jackals skywards. Here are some techniques to try (they're not so good on Elites and Hunters as they're heavier and won't fly as far). Once you've launched a victim, you can try shooting him whilst he's still airborne - they still bleed. Clay-pigeon shooting, cyborg style! The pistol is ideal for that; see how many hits you can get before he lands. More difficult is to get him with a rocket, or with a plasma ball from a plasma pistol.
Single tag
Tag him low down, e.g. on the feet or rear end. It's pretty easy when they're running away from you. When the explosion comes, he's liable to be launched way up high, and occasionally you might see one go almost vertically upwards. Incidentally, if you're facing a shielded Jackal you want to tag, ping a shot off his shield or circle him close up. He'll probably roll, which gives you a good tagging opportunity.
Double-tag
Tag him twice low down. With any luck the first grenade will blow him up high, then the second will shoot him even higher. Personally I think this should be an Olympic sport where they award points for artistic merit and stuff.
Tag and needle
Tag him low down then quickly send a stream of needles his way. The needles continue to home in even when he's airborne, and it's possible to get a pink explosion in mid-air. It's not easily done, but you should get a ten from the judges!
Tag and rocket
Tag him low down then rocket the ground beneath his feet. The rocket blast sends him up, and a moment later the grenade goes off, perhaps shooting him higher or sending him off in a different direction. You can send him for miles this way. As a variation, you could delay the rocket so it goes off at about the same time as the grenade, effectively getting just one very big bang. But your timing would need to be good.