Something I've been doing lately is freezing fresh-squeezed grapefruit or lemon juice into ice cubes (straight, not watered-down). Add 1-2 of those to a bit of soda and booze and it's a nearly instant cocktail, very refreshing, and easier to transport than a blended frozen drink. You can even freeze things like mint leaves or fresh fennel into the cubes to give it flair and added flavor.
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http://www.food52.com/recipes/4234_mint_limeade
It's not frozen but, if kept in a cooler, it would be pretty refreshing.