Almost everything worth doing on Sanibel is free. The beaches are free. Shelling is free and requires no equipment. The interior trails are free. Walking the lighthouse grounds is free. Watching the sun go down into the Gulf from any west-facing beach on the island is free, every night, with no boat and no ticket. What is not free is standing your car next to any of it.
That is the whole economics of a Sanibel day and it is worth stating plainly, because a lot of budget guides for this island quietly skip it. Every beach park on Sanibel charges for parking by the hour, paid at a machine or by phone, and enforcement is real. Budget roughly $5 an hour and assume the rate is reviewed periodically. Spend six hours across two beaches in a day and the free island has cost you more than the refuge entry fee.
The solution is a bicycle, and on this island that is a practical answer rather than a wholesome suggestion. More than 26 miles of shared-use path run the length of the island, from the lighthouse in the east out to Blind Pass in the west, physically separated from the road rather than painted beside it. There are no traffic signals on the island and the speed limits are low. If you are staying in a rental with bikes included, which many are, your parking costs for the week drop to nothing. If you are not, a week's hire usually costs less than three days of beach parking.
The second money-saver is the refuge. Wildlife Drive charges per vehicle, but a cyclist or walker over fifteen pays about a dollar, and children under sixteen are free. Riding the four-mile drive is also a better way to see it than driving: you hear things, you stop where you want, and you are not watching a road. The Bailey Tract, a separate freshwater parcel off Tarpon Bay Road with about three miles of flat trails, costs nothing at all and is open on Fridays, when Wildlife Drive is closed.
The third is timing rather than money. The best things on this island happen in the first two hours of daylight and the last hour before dark, and both are free. A falling tide at six in the morning on a Gulf beach is the single best experience Sanibel offers, and the only cost is the alarm clock.
What follows is what is genuinely free, what is nearly free, how to handle the parking honestly, and the few things where paying is the right call.