I don't want to use anything but the stable toolchain for my normal work but would like to be able to debug my macros using cargo expand. I don't even now how to use anything but the default stable toolchain.
You can install nightly rust with rustup toolchain install nightly, then you can run cargo as nightly with rustup run nightly cargo [...more arguments here] or cargo +nightly [...more arguments here]. If you want your normal cargo command to be replaced with nightly as well then you can run rustup default nightly.