Hey, I am almost at level 80 and I was wondering if I should do the 5 boxes event. What do I gain from it? Someone told me about mounts but other than that, why should I grind it?
You can get sonic springs which you can use to craft furniture based on the 5 boxes event. Each requires one sonic spring. I would suggest buying all of the recipes that interest you, because sonic springs are occasionally rewards from daily assignments.
Permanent fairy wings mounts drop from the box in Avalon.
Couch potatoes and deadly magma peas drop from some of the boxes.
You can get the Greenwarden's gear from each box. The stats are not impressive, but the gear is nice looking. The Energetic set gives extra energy, with more for the higher level gear. The full level 80 set gives 53 extra energy.
Teleporters, music players, and other furniture drop from some of the boxes.
A few pets are possible drops from the Avalon box.
If you want specifics, you can search through the pages on the wizard 101 wiki. Click on the bosses and it will show a page of possible item drops.