Interesting. Whenever I try to gain 100% advancements, I end up doing the "use up a full diamond hoe" achievement, and give up.
That isn't really that difficult to do; just find a large grassy area (about 40x40 blocks) and hold down right-click and run around and it will be used up within minutes. A bit wasteful but a couple diamonds is nothing.
- Mine a total of 2 million ore (currently at 1.96 million), half a million iron (currently at 479,000), and fill up 2 double chests with gold blocks, which will take another 2-3 weeks and a month respectively, not exactly sure about the gold but there is less than one empty row left in the second double chest.
- Explore all land in the current map I'm exploring, the 6th one I've made in this world (all max zoom, 2048x2048 blocks).
- Longer term - explore the underground across an entire continent, as they existed in 1.6.4 (depends on the exact size but I've reached ocean to the north, northeast, west, and southwest).
Those are some that I can think of; I am not playing this world with any real goals other than just how much caving/mining/etc I can do, which is just for fun, and any goals just come up as I play, with my last major milestone in this world being walking a distance equivalent to the old Far Lands, which I only thought of a few months before I reached it; in my most recent "other" world I did have the goal to find at least one of every type of cave/underground feature, which took about 20 days of playtime over 4 months and I stopped playing on it after finding all of them but no such goal exists in my first world, which has close to 120 days of playtime in total so far.
One thing for sure, I've long since reached my goal of having the most caved-out world anybody has ever made in Minecraft, possibly even if you include multiplayer servers with no map resets for years:
Those were made after I used a utility on a copy of the world to delete all chunks without torches - all of what you see has actually been explored (I modded mineshafts to remove their naturally generated torches, all 3 strongholds were found long ago) and the cropped world measures 6400x5680 blocks (the projection on the left side near the bottom is along a railway I recently made to a new base).