2 parts vodkaStir in whatever container you have to hand and pour over ice in a chilled glass. Sip it while waiting on your friend to come back with a box of freshly steamed crab.
1/2 part grapefruit juice
2 drops grapefruit bitters
2 parts tonic water (or to taste)
1/2 part Tres Agaves margarita mix (to sweeten it a tad - grapefruit juice, tonic water and bitters funnily enough made it really bitter)
Yes, it's going to be hard to leave.