Tell them to go to BGT so your stepdad can get his fill of coasters. They can take a shuttle bus from Orlando if they don't want to drive there. I've used this shuttle several times and never had any issues with it. The stops I used were twice at the old Mercado shops location on I-Drive and once from SeaWorld.
My husband and I love going to Universal Studios and Islands of Adventure.
When I was younger I couldn't get enough of IOA but now that I'm older we really spent alot of time over at USO.
The new Simpsons Springfield part was fun to visit and we spent time at Moe's.
Plus I'm a big Despicable Me and Shrek fan so that's the place for me.
The Beetlejuice show is very much on my list of must sees when visiting as well as the Horror Make-Up comedy show. I got picked as a victim and it really was a hilarious experience.
I know you said that your step-dad wants roller coasters but I really think that USO has more things to do and see while IOA has the big thrills but once they are over there isn't much left except Simbad for the shows. USO has more shows which is a plus.
Hope they have fun with whatever place they choose. All the options (SeaWorld, USO, IOA and Epcot) are wonderful places to visit.