I love Sloppy Joes!!
I've had them at a picnic, it's easier for the host because you can made them then put them in a crock pot and leave them on Warm and you don't have to worry about burning them on the grille!!
As far as them being messy, get paper bowls and people can just eat them out of the bowl with the bun on the bottom of the bowl!!

I like making different things when I have a cook out, theres nothing worse than eating hot dogs and burger EVERY weekend in the summer.

I vote Sloppy Joes!