As with everything else, there are antiques and then there are antiques! If they like auctions. the best and closest to our area is
!antique Auction Calendar - Burchard Galleries. Burchard Galleries deal in very high end antiques and are known world wide for their quality. When we were in business this is where we bought. If, however, they like to browse and hunt through "stuff" then Rennigers is a really fun day out. I would also suggest Mt. Dora, some decent shops and a really pretty town.
You can also google antiques in Central Florida and you will find several places around here and Orlando.