Michael Data

Sometimes giant cruise ships stop by that are bigger than the island. Key West is made to handle those giant waves of people, so there are a lot of tourist shops selling “lobster-trap” stuff.

Any Time of Day

Flying Monkeys Bar has the best daiquiris with a long menu of options, and at least one non-alcoholic slushy for kids. It's a bar facing the street outside a more formal restaurant. Good place to stop before spending time outside in the heat, or as a way to get out of the heat.

Plan Ahead

Any watersports or boat rentals: decide ahead if you want to do them so you can make the arrangements.

Possibly-free Parking

Smathers Beach is a nice beach, and not the most crowded.

Dog Beach is a tiny, secluded beach with easy street parking.

Bridge

During your commute: try to find part of the bridge with an open pedestrian walkway. I don't know which are open when but keep an eye out. Be careful, you have to carefully park in usually a dirt parking area outside it! They seem to mostly be on the west side of the keys.

Sunset

At Mallory Square, there's a sunset celebration and typically street performers and a friendly crowd. Info here.