I have a cute little Shepherd druid kobold with booming blade. Can I shillelagh as a bonus action and the action booming blade? Or is it only one cantrip. I know only one spell is permitted but does that go for cantrips aswell?

That works a little bit differently.

So, on your turn, if you are going to cast a spell using your bonus action, you can only cast another cantrip with casting time of one action in the same turn.

In your particular case, yes, you can use your bonus action to cast Shillelagh and then use your action to cast the cantrip Booming Blade.

Yeah, the rules for casting multiple spells per turn trip up new players a lot.

Basically, the bonus action is the main thing to check. If the bonus action is any spell (including cantrips) then the action(s) can only be a cantrip, and you can't cast as a reaction until the turn ends. And this rule applies retroactively, so you can't cast a leveled spell as an action then cast as a bonus action.

But you can cast as much as you want as long as it doesn't use your bonus action. For example, you can cast fireball, counterspell a counterspell, then action surge and cast a second fireball.

And your example works because booming blade is a cantrip.