I agree with coach bag fanatic. Whenever I've looked into the one day deals at the Dolphin they haven't been worthwhile. Really, Magic Express is quite nice. Before that, we rented cars. Otherwise ther are towncar serviecs that aren't too expensive. Less money that what you propose, and far less time consuming.
As for your original question - another option is hitting the pool and taking your time at the resort. 9pm is pretty late, so it would be worth going to a park...but you could also do DTD combined with swimming. There are locker/bathrooms rooms at the pool at Pop, and even another set at the 80's quiet pool (I don't know about the 50's pool. It may have them too, I didn't go over that way.)
Also, I wanted to point out to you that you don't have to decide now. You can decide that day (or maybe the day before) what you feel like doing. You can modify your MYW tickets at the front desk - you can also do it at the park entrance. (though there is sometimes a line there inthe am. The best time to modify tickets is in the pm.)
STill another option is seeing a movie at DTD or going to Disneyquest. If you like videogames, Disneyquest is paradise. (In our family, the guys often go to DQ while the women/girls shop.)