Our family did the airboats many years ago as kids. They were fun.
My first reaction was to say that there aren't any (to respond directly to the thread title) any other MUST do's in Orlando - but there are a great number of non Disney things to do in the area - and beyond. If you include all the day trip options, then Florida is a huge cornucopia of interests. I'll give youa few oddball ones that I liked.
Once when we were kids, we did glass bottom boats rides. I don't recall where we rode them, but they were WAY cooler than the airboats, esp. if your family can't do scuba. WIth a quick search, I found several places in FL that offer boat rides.
Indian River is just south of the Orlando area. Touring groves is really fun. Florida also had a ton of farms. Even in the Orlando area, is a great roadside orange stand that's in the shape of giant orange. It has nice fruit, too. It's up on 192, and it's been there a LONG time, when most folks stayed offsite. (website says it opened in 1973.)
There are LOTS of little places like the Aligator farms, parrot places - other animal attracions. Just off 192 is alittle place called World of Orchids. It's nothing too exciting, but if you like orchids, you can pick some up for a good price. They also will tell you how to care for them. (Again, I think they are still there.)
St. Augustine is a fantastic place to visit. I think it's about 2hours away, so we always stopped for the night there (on the way to Orlando and Ft. Lauderdale.) St. Augustine was once very Spanish, and it's VERY old. Many American history books tend to skip over the history of FLorida, but St. Augustine has preserved a really neat Spanish fort. It's also got Flagler College, neat shops. The whole town has a very unique look. It's a really great place to take kids. It's always a favorite with our family.
Daytona has the race track, but teh 500 is in Feb. Still, they have various events. It's a nice beach,and hey - you can still drive on the beach. I've never done so on a motorcycle, but it sure looked fun!
In the winter months, there's a considerable weather difference between Orlando, and Ft.Lauderdale. Ft. Lauderdale and vacinity is much better for beach going. (about 3.5hours south, so probably best as overnight stay) Even through March, you can get some chilly days in Orlando. the towns tothe south of Orlando all have unique personalities. Lots of great shopping. In Ft.L., one cool thing to do is to take a boat tour of the waterways on a tour boat called the River Queen. You can see how the top 1/2of 1% of America lives. (or at least a peak of their huge mansions). Lots of great dining and pretty beaches. Then there's Miami's Cuban personality, Hollywood's beautiful boardwalk (top notch!). A ton more parks and attractions in that area, too! Oh, and lots of REALLY nice cars! (no harsh winter to spoil them).
Further south is the Everglades, the Keys - but that's going to be way more than day trip from Orlando.
In the Orlando area, there are some minor attractions beyond the theme parks. DH once had fun doing indoor skydiving (you ride over a huge indoor fan). There are also go carts, mini golf, giant bungee, and others, but usually we find a week at WDW to be a better way to spend our time. There's enough at WDW to keep us busy for a week - much of what's offsite - like go carts, see a movie, shop an outlet mall - are things you can do in other places (esp beach towns)- though maybe not where you live personally. If you go beyond the immediate Orlando area though, the options in FL are nearly limitless.
These are all wonderful ideas. Thank you so much. I am putting my teens to work researching your ideas on the net so we have a list of options to consider when we are there. 
