Something I just picked up in the Steam sale was Tadpole Treble [official site]. It's an early access cartoonish side scroller where you negotiate a track in the form of a musical stave. I found it just by idly entering frog-related search terms into Steam the other day and so far it's been delightful.
The basic idea is that you get hurt if you run into the notes of the melody, but you can activate or collect other markers to boost your score, repair your health, or earn a tadpole power boost (sort of like an invincibility star from Mario but you can trigger it at a time of your choosing once it's powered).
It's simple at first but the tracks get more complex as you guide your tadpole, Baton, along the waterways and back to her home. I'm replaying some of the earlier ones in order to up my rating and haven't even looked at the composition mode yet.
It's an early access game so as usual I'll add the caveat that you shouldn't buy this unless you're fine with it being in this state and that any number of things could alter its development from this point.
At the moment it's £5.24 on Steam (usually £6.99).