While most of the course creation parts in Super Mario Maker 2 are unlocked from the start, there are two that you can’t immediately access. You’ll definitely want them, though.
One of Mario Maker 2's new features is Story Mode, in which you play through over 100 levels designed by Nintendo. This mostly stands on its own and doesn’t have any direct impact on the Course Maker level-editing tool, but once you finish repairing Princess Peach’s castle by collecting coins in the levels, you’ll get two new power-up items for Course Maker.
The first is the Superball Flower, a power-up from the 1989 Game Boy game Super Mario Land. It can only be used in the Super Mario Bros. theme, and it’s found by dropping a Fire Flower, then pressing on it to bring up the sub-menu.
Once Mario gets the Superball Flower, his sprite colors will change to shades of grey, and the stage music and some sound effects will change to Mario Land style as well.
The Superball Flower shoots a bouncing ball, perfect for getting into small crevices and defeating enemies.
Sorry if you were holding out hope that Super Mario Land might end up being a theme in and of itself in Mario Maker 2; I sincerely doubt that Nintendo would use its main unique gameplay mechanic as an unlockable bonus in this way if it was planning on making this its own theme.
The other unlockable power-up is only available in the Super Mario 3D World course style, and it’s the Super Hammer.
This transforms Mario into Builder Mario, who can swing (not throw) the hammer to smash blocks, including the otherwise difficult-to-break Hard Blocks. [Update, 6/28 1:10 a.m. ET: I am reminded by a commenter that Builder Mario can also, out of thin air, generate crates that he can stand on. Important!]
Both of these power-ups make some unique course designs possible, so they’re probably ones you’ll want to unlock as soon as you can for your adventure into Mario making. Fortunately, Story Mode is a lot of fun, so finding these will be a pleasure. You needn’t play all of Story Mode to get them, just enough to rebuild Peach’s castle.