First off, just how senior? By some measures I am a senior (my local pharmacy keeps foisting the damn discount on me!), and am quite happy to go to Disney or Universal (or both) all week long. As a matter of fact, if your touring style is slowing down, more shorter days at the parks makes much more sense than one longer exhausting day. If you are only going for one day, you will likely only be able to do one of the four Disney parks.
After that, the question is what does your group of seniors enjoy?
If you are staying at a resort, there is relaxing by the pool, and many resorts now offer pickle-ball which is very popular. Orlando is a great shopping and dining destination. The theatrical dining experiences can be fun. A road trip to the beach can be great, you are about an hour away from both the left and right coasts of Florida.
There are a ton of "B" attractions as well. Gatorland, Holy Land Experience, visit an orchard, take an airboat ride on the everglades. Seaworld has lots of viewing attractions; not as thrill ride oriented as many of the other parks.
When we go we always swear we will do a couple of down days at the resort, but that almost never happens. There is so much to do in Orlando/Central FL that we are always saying "darn we should have gone to..." on the long drive home as we plan for the next trip.
ETA: +1 on Blue Springs. A great attraction in the winter.