Book four things in advance: flights and accommodation, a hire car if you want one (they sell out in summer and at New Year), paid trail permits at simplifica.madeira.gov.pt for routes such as PR1, PR6 and PR9, and any sunrise or whale-watching tour. Everything else — restaurants aside on Fridays and Saturdays — can wait.
Hire cars are the single most common thing visitors leave too late. Madeira's fleet is finite and demand spikes in July, August, Christmas and New Year, when prices roughly double and small automatics disappear entirely. Book as soon as your flights are confirmed.
Paid trail access is the second. Several of the island's best-known routes — PR1 Pico do Arieiro to Pico Ruivo, PR6 Levada das 25 Fontes, PR9 Levada do Caldeirão Verde among them — need a ticket bought online before you walk, currently €3 for most routes and €4.50 for PR9. Buy the night before at the latest and re-check official closures the same evening, because storms and rockfall shut sections at short notice.
Tours worth pre-booking are the sunrise runs to Pico do Arieiro (they leave Funchal around 04:30 and fill up), whale and dolphin trips in peak season, and the Porto Santo ferry in August. Restaurants generally take walk-ins, but the well-known Funchal and Câmara de Lobos places want a reservation on Friday and Saturday evenings and around New Year.
You do not need to pre-book the cable car, the Monte toboggan, the market, museums, or bus travel. Buy an airport transfer only if you land late at night — the Aerobus is €6.70 onboard or €5.55 pre-purchased, and taxis to central Funchal run about €25–€30.