Minecraft wiki entry for The_Void, associated text: Store items in an ender chest or type /gamerule keepInventory true, then kill your character. As long as there is another ender chest placed (and all items are stored), no items are lost. However, the player should keep in mind that they will lose their experience if they do this if they did not enable Keep Inventory.
Build a portal on the Nether's top and go through it. Keep in mind the player could die of fall damage, if very high up.
Use a glitch to break bedrock and get back down from the Nether roof.