I'd say if you're into the carbonation - try Pellegrino or (for a BIG carbonation kick) plain seltzer water (like Canada Dry or Schwepps) with a lemon or lime squeezed in.
It's a GREAT idea to kick the sugared soda habit. I remember back in 1985 during the "New Coke" hoo-hah - that month or two before Coke came to its senses and brought back Classic Coke - that there was a gal here in the Bay Area who was part of the anti-New Coke clique here who was in the news constantly (I believe the parent organization was called the Old Cola Drinkers of America). This gal who was on the hefty side said that prior to New Coke, she was drinking either a 6 pack or 12 pack of regular Coke each DAY. Of course once Old Coke was no longer available, she quit cold turkey and before Classic Coke was reintroduced, I remember she lost somewhere in the area of 25 or more pounds!
So those liquid calories can really add up...
Anyway - I enjoy Diet Cokes myself, but I also know that it's VITAL for me to get in my H20 on a daily basis. So what I do is NOT drink sodas at work. I have four 24-oz water bottles that I refill every day at the water cooler - my goal is to empty those bottles between 8 am and 5 pm, plus in my morning workout I drink another 24 oz of water, also if I have a Pilates or Yoga class (2-3 x week) I drink yet another bottle during the class. Diet Cokes I limit to one or two a day. (Actually our latest taste thrill at home is the Minute Maid or Tropicana Light Lemonade in cans...those are the bomb!)