Board Thread:Plants vs. Zombies Heroes/@comment-27769607-20170803032920/@comment-24471175-20170808201832

Sandbox Mode:

For this mode, you select both a Zombie and Plant hero and deck, then play as both heroes at the same time with infinite health, sun and brains. There's also a "deck" icon where if you drag a card in your hand onto it, it shuffles it back into your deck. If you tap the deck icon, you draw a card from your deck.

This basically lets you experiment with cards/combos or try mechanics, but anything you do in the sandbox does not count towards quests.