How much of Florida have you (or Dad) seen outside of the theme parks?
St Augustine is always a good recommendation. If you drive south from Orlando toward the Keys, I would stay off the interstate. Take the scenic route & stop at Bok Tower, Lake Okeechobee, & the Everglades. Florida has a lot to offer beyond the beach & theme parks, but it's hard to offer recommendations without knowing what kinds of things you are interested in. (Carillon concert or airboat tour? Kayaking through the mangroves or swimming with manatees?)