I LOVE seltzer, especially the flavored ones! My favorites are Polar vanilla, grapefruit, black cherry, and pomegranate. Last summer they had a pineapple-coconut - I really hope they make it again this summer!
I drink club soda a lot when out with friends - the bubbles make me feel like I'm drinking something much more interesting than water.
ETA - club soda only has 10mg sodium per 8 ounces, so I don't think drinking it occasionally will hurt.