I would say that most everything at Hollywood Studios is out; even Toy Story Mania whips around a lot, and I was embargoed from that by my surgeon for a year after my back surgery.
Most of the boat rides (save Pirates & Frozen?) should be fine; so that's Small World @ MK, Living with the Land & Gran Fiesta Tour @ EPCOT, and Navi River at AK.
There is still so much to see & do at WDW if you take the focus off of rides; most people don't know that EPCOT World Showcase has several really cool hidden little "museums" with fascinating displays about the country/pavilion where they are located. Arribas Bros has crafts people working "on stage" in every Park except AK, and at Disney Springs too, and in the confectionary you can watch Cast Members at work also. For every Hall of Presidents and Carousel of Progress, there will be something equally cool or fun or fascinating. If y'all are there during Food & Wine, EPCOT will have so many food booths, each with little sampler plates to share and try (extra fee). Even for those under 21, there will be plenty of interesting beverage options; many of the booths have "virgin" drinks.
Because she's 18, she may actually enjoy spending time at Disney Springs as well; in addition to a Starbucks Reserve (Ask the CMs what it means!) there is *tons* of shopping and great food as well. She could (for an extra fee) even ride in an Amphicar! (which should be smooth enough)
Don't forget Resort Hopping - if nothing else, y'all should grab the Monorail, and ride around the Resort Loop; you will pass right through the Contemporary (They have a couple of great gift shops + the famous Mary Blair Murals, including the famous 5-legged goat!) and it's always fun to grab a snack there as well. You can ride on around to Polynesian (the only place outside of the theme parks where you can buy Dole Whip!) and the the last stop before you complete the loop would be Grand Floridian, which is just too pretty to miss. Some folks even do a "progressive" Monorail dinner, where you have an appetizer at one resort, your main entree at the next resort, and you end. your Monorail evening with dessert at the last of the 3!
There's also the EPCOT Loop, and even if you don't want to get off the Monorail, it will still take you through the Park; you get a bird's eye view of everything, and it's a chance for some great pictures of Spaceship Earth.
There are boats to ride (free); not just the Ferry Boats from the Magic Kingdom parking/TTC, but also there are launches that run to Poly, GF & WL across the Seven Seas Lagoon, and Bay Lake, as well as a boat that goes out to Fort Wilderness. For an extra fee, you can go to the Hoop Dee Doo Revue (a personal favorite!) and enjoy the longest-running live show at Disney World. The food is really good too - I could live off of the cornbread alone! You can also visit the stables at Fort Wilderness, and meet the Disney horses. You can also take the Friendship boats that run between Hollywood Studios and the International Gateway at EPCOT.
At both HS & EPCOT you can also grab the Skyliner (free) for a lovely roundtrip across WDW; you will have birds-eye views that you can't get any other way!
Most all of the Resorts have evening campfires with free marshmallows to roast (bring your own supplies for S'mores!) and many of the Resorts have daytime craft activities that range from tie-dye to painting. Visit Port Orleans French Quarter for the famous Mickey Beignets (extra fee) and you can walk or take a boat from there up the river to their sister Resort Port Orleans Riverside, where the world-famous Yee-Haw Bob entertains several nights a week; the show is free, food & drink is extra.
Grab a bus and head out to Animal Kingdom Lodge where you can visit the animals on the Savanna, and take in one of the greatest collections of African art outside of country. AKL is worth the trip just for the buffet at Boma!
So... yeah. There's tons of stuff to do at WDW that doesn't involve any ride more exciting than a bus or a monorail, and I have just scratched the surface. (and sorry this was so long!)