Moonbeam is pretty decent value for a single 2nd level spell slot.

A single cast on a sole target will normally net you 4d10 (save for half) damage - remember, the beam does the first 2d10 the moment it is applied on the enemy, and another 2d10 the moment the enemy starts its turn. If you can line up 2 targets or more, then you're looking at 8d10 damage (save for half) damage in just the first round. It's has fantastic range (horizontal and vertical) and it's undodgeable - you're always guaranteed some damage and it's perfect against annoying high AC enemies.

Add to the fact that this is a concentration spell you can apply over multiple rounds (for just the 1 slot), the value is quite insane as long as you can keep the concentration up.

If you want to be very cheap, Moon Druids can take advantage of wildshape to guarantee concentration for at least a few rounds. Moonbeam -> Bonus action Bird form -> and fly out of range of any danger for the turn (maybe even into some very advantageous position). From there, you have tons of choices. If the position is sufficiently safe, you can just moonbeam away from the safety of high ground and superior line of sight. Alternatively, you can move the moonbeam, and repeat the bird form maneuver to guarantee another round of damage.

I don't think Druids have the explosive power of some of the other classes, but its versatility and sustainability is a perfect complement for your nova classes.