Ride Journey to the Center of the Earth and 20,000 Leagues Under the Sea back to back when both are on your list. Leaving the area and coming back later costs time you will not get back. The queues here do not get shorter as the day goes on.
Expect long standby waits on any day with real crowds. Journey to the Center of the Earth is one of the most in-demand rides in the entire park; standby often runs 60–90 minutes or more. Realistic options are an early rope-drop run, or Disney Premier Access (the paid skip-the-line product, bought in the Tokyo Disney Resort app). Confirm current pricing and windows in the app before you go, because they change.
Eat and shop in the same stop when you can. Vulcania Restaurant, Nautilus Galley, the snack carts, and Nautilus Gifts all sit inside the volcano loop, so you do not need a second trip for food or merch.
The volcano is one of the easiest landmarks in the park. If you can see Mount Prometheus, you know which way Mediterranean Harbor and American Waterfront are.