Is there anything near Legoland (FL) that would be appropriate for older kids to do while the younger ones enjoy Legoland? Is Legoland a full day park? TIA
Starting from the back, Legoland is a full day park, but it's a short full day. It's usually open from 9/10am-5/6pm and you can find entertainment to fill all those hours, especially depending on your child's age (and that doesn't count heading into the water park if you get a 2 park ticket).
As for things to do near Legoland, there are lots of nature activities, but I've never done them - there are boat rides, fishing, paddleboarding, etc. I don't know that I'd send a 16 year old to do those solo, but you could.
If you bring your 16 year old to Legoland, I would suggest planning on letting him wander some hours solo. The Old Cypress Gardens area and the Miniland area are gorgeous for nature and
Lego lovers alike - you could spend a couple hours total enjoying them both. If he gets sick of these, there is a video game try out room in the back of the park where he might enjoy spending an hour or two. He might also enjoy a few attractions (the boat driving, the 2-3 biggest rollercoasters - they are Big Thunder Mountain level, and maybe the 4d movie). Will you get your "money's worth" for his ticket - maybe not, but tickets can be had really cheap, so you won't lose much.