Clams. Have you ever grilled clams? It's ridiculously easy and all you have to do it sit and wait for them to open. Then you can toss them with butter or throw them in some linguine if you want. If you want to up the ante and do oysters, make sure you save the liquor in the shell.
Haloumi is the reigning grilled cheese (not sammie, actual cheese on the grill), but Benjamin Roberts once told me that a nice fat hunk of provolone could do with a nice char as well. Also, you could smoke some brie on a solid cedar plank if you've got one.
I am very psyched to try grilling my limes for caipirinhas, mojitos, margaritas, beer, pork tacos, you name it. Let's char those green meanies.
I love the idea of grilling mushrooms, not as a portabella faux-burger, but as a smokey, snacky, eat with fast fingers faux-chip.
If you're having a small, special gathering, consider grilling lobster halves. At Coastal Seafoods it's $16.99/lb (cheaper than Copper River Salmon!) and there is something so lush about digging into the smokey charred shell and just going to town. Maybe there's a little corn on the side, maybe a little bubbly in your glass. If you want to keep it low-brow or have a bunch of people to feed, consider grilling prawns which can be a ton of fun for their messy deliciousness.
You already know about Mark Bittman's grilled melon obsession, right?