Basically a Mini-Land Mayhem level pack plus a level editor; but for $20 you could do a lot worse. The levels packed in with the game have a weird difficulty curve where the majority of the main game's levels are extremely easy, the extra levels and first 15 or so bonus levels reasonably challenging, then for the last 10 or so bonus levels they went mad with power and made the most obtuse and difficult levels imaginable.
The level editor itself is extremely intuitive, robust, and fun to use. Thing is though, so much of the game's content was online and not only is that shut down now, but even when it wasn't, it's a kids' game focused on user created content. In other words, the vast majority of what you'd find was trash. At the very least there were some Nintendo created levels that were uploaded and were fantastic, but they're lost media now. You can at least find pictures of them online and recreate them in the level editor like I have.
The level editor itself is extremely intuitive, robust, and fun to use. Thing is though, so much of the game's content was online and not only is that shut down now, but even when it wasn't, it's a kids' game focused on user created content. In other words, the vast majority of what you'd find was trash. At the very least there were some Nintendo created levels that were uploaded and were fantastic, but they're lost media now. You can at least find pictures of them online and recreate them in the level editor like I have.