Cheap and cool and would be fun at an outdoor summer party. Available here for only $16.
"A glass that looks like it is filled with milk, but in fact, the inside has a special coating that lights up when you flip the switch. Now, you only have to flip the switch once though, because if you don't want it to be on anymore, just turn it upside down and voila, it's off... Perfect for a nightlight, or next to your bed or in the bathroom, or for the kids or - you get it, there are a ton of usage occasions that you can come up with."