Manage timing of arrival and flights vs boat is all...check schedules.

Rental bikes worked very well for us. Rode to town and back a couple of times.

You could also take a ferry boat. I recommend flying.

We stayed 7 miles north of town and at the time the road did not go that far north so we did not have a golf cart. Renting a cart really depends on where you are staying but with the road built now you can certainly do more exploring with one.

Definitely snorkel Hol Chan and Shark/Ray. We stayed close to Mexico Rocks so we did that as well.

If we land at noon, what time do I book for the flight to San Pedro, or ferry? Should I leave a lay over of 1 hour or more than 2?

I can’t remember how long we had. Our friends had done it before so I think they booked for us.
And make sure to hit the duty free before you check in with Tropic. We filled two boxes with liquor and checked them. Saved us a ton of cash over in San Pedro.

I've done that flight many times as I use to cover Belize for work.

The airport is tiny, so it's more a matter of whether or not your in-bound flight is on-time.

Because the flight is only 15 minutes, it runs multiple times per day. Worst case, you take the boat but it can be rough at times.

Ambergris Caye is great. Be sure to eat at Elvi's Kitchen.