Necessary Combinations:

Shop - Apple + Banana

Guess the Words (mini-game) - Potato + Lemon (The list of word definitions is located in the guide)

Sale in the Shop - Banana + Cabbage

Magic Mushrooms - Cherry + Avocado

The fruit combos for level skips (in no particular order):

Grass Land - Apple + Avocado

Temple Land - Banana + Mango

Lunar Land - Orange + Cherry

Wood Land - Potato + Potato

Mine Land - Potato + Cherry

Snow Land - Potato + Avocado

Dead Land - Cabbage + Pear

Desert Land - Mango + Onion

Other spells that are useful/necessary:

Slow Time - Banana + Mushroom

2x Time Icons - Mango + Mango