Maybe a land of mushrooms sounds more like your cup of tea. Well, have a cup of tea while you collect the 64 sponges in this map then.
My Sponge Challenges tend to be mostly Open-World, and the actual worlds are limitless, though the sponges usually stay within a few hundred blocks of the spawn point. Normal survival rules apply, meaning you can place blocks, break blocks, craft things, and get yourself killed by the various monsters and such. You should also keep your inventory upon death, so you won't have to worry about losing any of the 64 sponges if you keep them with you.
My Sponge Challenges are all fairly difficult, so be prepared to die somewhat frequently. Use any strategy you can think of, other than using cheats and setting it to peaceful. Playing on Easy, Normal, or Hard is required for this map.
Unlike most CTM maps, you do not use the sponges to complete a monument, the objective is just to obtain them. Other than this difference, they are essentially CTMs.
Shrooms was made without the use of any external world-editing programs.
Feel free to leave a comment with any suggestions you have, and I might implement them in future Sponge Challenges. Note that some later Sponge Challenges are already made at the time of this map being posted, so you might not see your suggestions implemented right away. I might also release updated versions of existing maps with your suggestions in place, or you could just tell me what you thought of the map in general. Feedback is always welcome.