Well, you could always, you know, eat them LOL
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)
Pat