Things to do when Minecraft gets too easy

I see a lot of people complaining that Minecraft gets too easy once you've built a fort, even saying you can be "totally safe" the first night. If you're tired of the challenge of starting new worlds, here are a few ideas for what to do when Hard has got too easy. Add your own in the comments, I would love to try them!

Spelunk This seems obvious but some people have really never tried it! Natural cave systems are full of resources, but they're also full of twists and turns that hide the evil monsters within. Invading an awkward underground room full of skeletons can take days, even with all the torches as you can carry.

Expand Congratulations, you've built an impenetrable fortress! But hey, I have news for you: your world is 8 times the size of Earth. You're the king of nothing, pal. Over the next hill, nobody's heard of you. Go conquer that valley too. And the next, and the next.

Get Shipwrecked! Take a boat and sail until your W-finger gets cramp. Pay no attention to where you're going and don't take a compass. Land your boat at sunset and build your whole civilization without dying, because you'll never find your way back here. Bonus idea: Whenever you die, take off in another direction and do the same again. Then use your map, full of lost cities to discover and explore, for SMP.

Backpack Get as far as you can from spawn, and just see the sights. Only carry what you can hold in your action bar. Building a structure is against the rules; you can only hide in natural caves. Put your screenshots and your Cartographs in an album and wow r/Minecraft with how well travelled you are.

Take Risks Of course you're safe at night, indoors, surrounded by torches. Cuddling your mommy. Real Minercrafters spend the night building redstone computers in the dark.

Complain that Minecraft is too easy and Notch should make it harder Um, actually, please don't do this. Making the game harder for everyone would mean all of the above styles of playing would be impossible, and it wouldn't even benefit you. The reason you find it easy is you built yourself a safe place and don't venture far. Making the game harder won't fix this: Until mobs can dig through obsidian or spawn in daylight, you'll always be able to build a safe place and hide.

There are still challenges waiting for you if you go looking for them. Please use those opportunities and don't take them away from the rest of us!

Edit: Ideas from the comments:

Above the sky, beneath the ocean, inside the earth's mantle Build a fort somewhere deep enough to drown, high enough to splat, or burny enough to really sting. Entrance through the nether only!

Edit 2: postagedue's suggestions are brilliant, check them out.