Alligators, 20 foot pythons, and wild dogs are nothing compared to a swarm of mosquitos and horseflies when you have to walk 5, 10, 20 miles or more to get back to your car or at least somewhere you can hitch a ride.  A well-maintained bike with a good tubeless setup can only prepare you for so much.  Mud, sharp limestone, and overgrown levees can have you helpless in the middle of nowhere unless you pack the right gear.  I've done quite a few adventure rides in, around and across South Florida, along with a few organized 200-mile races.  My gear varies slightly between these two types of rides (adventure versus races), but much of it stays the same.  Here's what I pack to make sure I'm not walking for 3 hours or stopping every mile to patch my bike back up.

Standard supplies

Whether you're riding the road or a short gravel route, these are the basics and can be packed in a smallish saddle bag:

1. CO2, two if not three, plus adapter.  Practice with it.  Make sure your valve cores are tight, too.  Nothing like inflating a tire, then watching it all come back out when you take the adapter off.
2. A tube! It doesn't matter what tires, sealant, or pressure you run.  Something can rip your tires open and have you walking home.
3. Multitool. Make sure it has a chain tool (see below).  Doesn't matter if it's heavy, it's Florida, you're not climbing anything.
4. Tubeless patch kit. 
5. A tire lever or two.

Adventure supplies

For longer rides, where it might be 20 miles out of cell service or the nearest paved road, I pack some extras.  These fit into a small bag that I can tuck into a back pocket or a hydration pack.

1. Replacement derailleur hanger.  Mud or weeds can cause a catastrophic mechanical.  This is a small item that can keep you pedaling.
2. A single speed conversion pulley wheel.  There are a few out there, but the one I like is by Origin8.
3. Quick links.  You can have your chain already set up with two so that you can take out a small section to make the switch to a single-speed easier.  I'd still carry one extra for your riding buddy who didn't set up his chain.
4. Quick link chain tool.  Wolf Components makes a tidy little one that also has a valve core slot.
5. Zip ties.  Can come in handy for a number of reasons.
6. Duct tape.  Ditto.  Wrap around a pump or your seat post.
7. Pump.  3 CO2 cartridges aren't enough for 100+ miles.
8. A spare tire.  Seems like overkill, except this, saved my DK200 after my front tire ripped open in the first hour.  I also take one on my solo cross-Florida rides.
9. A disc brake tool.  Works for separating brakes, truing the discs, and opening a beer.

Gravel Extras:

1. Bug spray with high DEET %.  The organic stuff doesn't work.  Spray on your shoes, your pockets, maybe your pack to keep it off your skin if it irritates you.
2. Extra sunscreen.  That six-hour 100-miler always ends uptake 7.5 hours or more.
3. A water purifier.  Small, lightweight, can save your life if you get delayed by a few hours.