1. Cheap Awesome Food: In key largo right across from the main tourist attractions (Queen Mary Boat, Glass bottom boat, etc.) there is a Cuban restaurant/take out. Locals eat there. It's says Cuban Food or Cuba Restaurant...very basic. Lots and lots of food super cheap and homemade....Get the Yuca Root with Mojo Sauce....if you love Garlic.
2. Just south of Key Largo (I think it's Tavernier Key) on the gulf side is a bird refuge. Awesome displays, attracts a lot of non-rehabbing birds because of the food. It's a nice "boardwalk" type of walk through the place. 30 min. max on your way down will do it. Free/Donation.
3. Islamarada: Robbies Marina. Buy a bucket of bait fish and Feed the Tarpons. They are huge prehistoric fish. You'll see a great many. You feed them off of the dock. Short but sweet. The buckets of bait only cost a couple of bucks.
4. Just north of Marathon is the Dolphin Research Center. This is about $30.00 per person, but worth it. Get to see dolphins in their habitats and watch the trainers....they have live shows too where the Dolphins get to show off their tricks. Not a circus act! These are scientists who train and study dolphins. This is a few hours.
5. Bahia Honda State Park south of Marathon. Beautiful park and beach. I once read it is considered one of the most beautiful beaches in the world. Prolly a small entrance fee.
6. Deer Key. This is a semi rural key with lots of residents with large properties. Drive around and see the Key Deer. They were once almost extinct, are exclusive to the keys...are only 2-3' tall full grown. They are everywhere now since well protected. They are used to people, so easy to get close enough to get good pics. Nice drive through on way down. Free
7. Key west.....well, that's just full of stuff to do and see. I like the Fort, checking out cruise ships, and tavern hopping (a must). A cool collection to start is getting pub coasters. You can add to it in your travels and it's free. I think the best way to get around Key West is by Scooter Renting. Not cheap, but worth it to see more and not be stuck in traffic all day.
8. Ditto on the Mallory Square sunset...It's a big event every day!