My first suggestion would be, if you want her to ride other things, don't take her on Splash Mountain until the very end of the day! If you do it early, that final drop will probably scare her, and she won't believe anything you say for the rest of the day. She'll expect there to be a drop like that on everything, and she may think you tricked her. While Splash isn't a coaster in the traditional sense, that final drop is still pretty scary for kids who don't like coasters yet.
I would start her out on all the calm stuff -- Small World, Peter Pan, Dumbo, Aladdin, TTA ... work your way up to faster stuff like Pooh, Pirates, and Buzz. Then let her watch Splash Mountain from the outside, to see how many kids her age are riding it and having fun. Let her know that the last drop is the ONLY drop, and that it's a water ride like Small World, except for the fun drop at the end. Let her watch families coming off of the ride, smiling and laughing, and then see if she wants to go. If she says no, then respect that.
If you want to talk to her ahead of time ... pick up one of the "WDW for Kids" books -- the one that's written by kids for kids. Read her the descriptions of rides from that book, since they'll be more kid-oriented. Let her decide, but also let her know that whether she chooses "yes" or "no" on specific rides, she's allowed to change her mind once she's at the park and sees the ride. And then just let it alone. Don't keep trying to get her to change her mind or to agree to ride. The more you try to tell her how much fun it is, the more she'll wonder, "If it's that much fun, how come you're working so hard to sell it?"
Bottom line is that you'll have fun there no matter what, right? It's not Splash Mountain that makes the trip special ... it's the family being there and having fun. Not going on Splash Mountain (or any other ride) won't ruin the trip for her unless you let it.
