You can see a variety of terrain in this screenshot, including the type of variety the new noise caves can generate + lava rivers. Note that rivers are meant to be long and sometimes difficult to explore.
One thing I've noticed through my own 1.20.1 modpack playthrough is that the new terrain really plays off of mineshafts and dungeons now. The new caves are designed to adjust the pacing of your mining experience, and it shows.
A small river I found deep beneath the ground.
Some bugs are deliberately left in to increase the interest level of caves. They can lead to a number of waterfalls throughout the world, such as this one.
It's always nice when the carvers align to produce very complex terrain.
A small sample of what you can experience deep underground.
A long room with a lot of texture.