Turn on Round trip on the Directions screen to go to your destination and back with the same captain. Pick how long they wait for you there — no wait, 15 or 30 minutes, 1 or 2 hours — and the fare you see covers the whole trip: out, the wait, and back to where you started.
While you're away, the trip screen shows when your captain expects you back. Need longer? Tap Extend to add 15 minutes (up to 2 hours in all) — the fare updates straight away. Back early? Just start back; you pay for the wait you booked.
If you're back after the booked time, the extra minutes are charged at your ride type's waiting rate, for up to 30 minutes. With no wait booked, the first few minutes at your destination are free (3 on most ride types) before that rate applies. If you haven't come back 15 minutes after the booked time, your captain can end the trip there: you pay for the way out and the wait, not the way back.