They can be good added to fruit salad if people don't like them "straight up".
Juice them and make granita/ice/etc type thing.
Juice them, mix approx equal parts of grapefruit juice and soy sauce plus a bit of sweetener to taste (I use honey) and marinate chicken parts in it (preferably skin-on) and then broil or grill, mmmm.
Uh... they're also good if you sprinkle brown sugar on the cut surface of the halves and run 'em under the broiler.
'Sall I can think of offhand, other people will come up with more. Enjoy!
(e.t.a. -- if your weather is dry enough to do candied grapefruit peel, and you can stand that much sugar [Wifezilla, don't read this LOL], that's really really tasty IMO. Use the actual fruit part of the fruit for any of hte above of course)
Not like grapefruit?!?! Are my eyes working? Must be something wrong with this computer screen! That can't be right!
Just teasing. (Why is it the one person who can get fresh citrus in their csa box wont eat it?? Buncha weirdos!!!)
Anyway, you got some good ideas already. I have had pork with a citrus marinade and it was totally nom. Grapefruit would work for that. The jelly idea sounds good. You can always candy slices of grapefruit. You can also juice them and then mix the grapefruit juice with orange or tangerine juice.