I see two tours being offered on the RCI website. One is for a cruise around Biscayne Bay on a small tour ship, looking at the homes of the famous and infamous, South Beach, etc. The other is an airboat tour in the Everglades, which would be fun. It isn't listed as strenuous.
Both are listed as 4 hours total duration, so it beats spending that time sitting around the lovely (not!) Miami International Airport.