The Peaks Island ferry terminal is located on the corner of Commercial Street and Maine State Pier. Here is what to expect:

You buy tickets at the front desk window.
You need to specify Peaks. There are other ferries for other islands.
Peaks is usually the car ferry that departs out of gate 5.
You only need a ticket to get to the island, not returning.
It fits about 600 people and about 15 cars.
Yes, you can drive your car onto the island, but it is expensive.
If you are not staying on island, park in Portland. There is a garage next to ferry building, but it could be crowded. May be better to Uber or walk from hotel.
Ferry generally runs every hour but check schedule.
It is a 15 to 20 minute very scenic ride.
Don’t get surprised if you see a seal riding in the wakes.
There is indoor and outdoor seating on boat.
It could get cool on the water, especially at night.
Once you get to the island you will exit the ferry and walk about 50 yards in an orderly fashion to the Peaks Island Inn which is where the wedding is located.
To get back on the ferry, you return and line up by the live lobster shack. You get on once arriving passengers get off.

If you are staying on the island or just going to wander around, there is plenty to see and do. It is a great place to relax.

Rent a golf cart. There are two spots to do it just after you depart the ferry. It is the best way to see the island.
Rent a bike and explore the island with pedal power.
Have an ice cream at Down Front.
Park yourself on a beach and soak up some rays. There are two sand beaches. (a loose definition) One is by the ferry the other is around the corner at the end of Greenwood Street.
Much of the coast is rocky and you can watch the lobster boats or build a tower with the stones.
Visit the 8th Marine Regiment Memorial.
Take a long walk. The main road is paved and about a 4-mile circle with one steep hill to climb or go down depending on your direction.
Explore the World War 2 bunkers. If German U-Boats were coming this way, it would be one of the first places they passed.
Have lunch at Island Lobster. Maggie says that the lobster rolls at this place are one of the best she has had.
Have pizza at Il Leone in a little outdoor park area.
Take a swim in the Casco Bay and count that as your cold plunge.
Go to the umbrella cover museum. Yes, that exists, but please don’t ask me why!
Bring a beach chair and read a book while watching the waves crash against the rocks.