What other vegetables can be used in cake/muffin baking?
Ah, you missed the obvious: pumpkin!

This is biologically a fruit, actually, as is the zucchini.
Pumpkin loves same spices as carrot. Maybe even rum or butterscotch too.

Now, beetroot is a veg and colors cakes delectably; perhaps why it was originally added when going stingy on the cocoa.

How about parsnip next? Sweet and nutty, I think pistachios would complement it well in a recipe borrowed from zucchini bread.

more veg here:
http://www.deliciousmagazine.co.uk/articles/how-to-make-cakes-from-vegetables

p.s. if you haven't guessed Rabarber is rhubarb