I would think if you're wanting cocktails on the beach but are worried about sand or other logistical concerns, I would just batch the cocktail before you go and take it in some kind of pitcher, jug, etc. Easy ones would be daiquiri, marg, etc. An ideal vessel would be one that can seal shut so you can give it a shake when you serve (assuming citrus is involved).
If you want to make it on the beach, something of the highball variety would be great, as it's just pour and drink. Dark and stormy, rum and coke, etc.