Mike, what time of year are you travelling and what are the interests of the girls?
A few ideas are, Early morning/dawn balloon trip over the WDW area (the idea may not fly with teens as they are notoriously difficult to get out of bed LOL) , A helicopter trip, trip to the Kennedy Space Centre, airboat rides ( if you're going in summer the night rides give a better chance of seeing a gator), if you're going in winter/Early spring swimming with manatees at Crystal River, shopping is likely to be popular with two girls, the waterparks/pools always seem to be popular. There are a load of other nature based ideas/attractions in the area I can give you some ideas on those if they'd be popular. I've just bought a book about historic Florida which has some really interesting places to visit, some within an hours drive or so of Orlando (again with some teens this may not fly, others might be interested) final ( for now) idea is the Salvador Dali museum in St Pete's , for anyone interested in art this is a must see. It's a fantastic chance to see how one of the 20th centurys greatest painters grew throughout his career as the exibit is solely the paintings of Dali.