you just get in the plane
On every island, the Poptropica blimp is above Main Street. To leave Nabooti Island, return to Nabooti and walk right from the Museum.
with the phone you found in mountain of moons you ring Vince when you are talking to him then he will leave to find his phone and then you look in his bag
After you have placed the jewels back in the Nabooti totem, climb back on the blimp and select an island. (see related question)
I don't think you can leave an island with a horse.
any planet
All islands are available on the Island Map. Use the blimp from any Main Street, or from the oil rig on Super Villain Island. If you are in space on Astro Knights Island, your ship will park on top of the castle. If you have the Haunted House card from the Poptropica store, you can leave any point on any island by going to and then leaving the House location. If you return to an island you leave, you will normally be back on Main Street.
To leave, use the Poptropica blimp on Main Street. (To complete the island, see the related question.)
You can leave anytime by backtracking. To leave from the falling blocks or sand room, there is a passage to your left. (see the related question)
Wait for principal to leave then you leave
Leave the castle, enter the city, and jump up into the blimp.
You click on the boat near where you came
just leave the fair and that's that.