This is by no means a comprehensive list, but it includes some of the (in my opinion) easier items to obtain/make, and all are items the NPC loves. Alex - complete breakfast, salmon dinner

Elliot - duck feather, lobster, pomegranate

Harvey - pickles, wine

Sam - cactus fruit, pizza

Sebastian - frozen tear, sashimi, void egg

Shane - beer (once keg), hot pepper

Abigail - amethyst, pumpkin

Emily - amethyst, wool

Haley - coconut, sunflower

Leah - goat cheese, salad, wine

Maru - cauliflower, strawberry

Penny - melon, poppy

Caroline - summer spangle

Clint - amethyst, aquamarine, omni geode

Demetrius - bean hotpot, strawberry

Dwarf - amethyst, aquamarine, omni geode

Evelyn - beet, fairy rose, tulip

George - leek

Gus - orange

Jas - fairy rose

Jodi - vegetable medley, chocolate cake

Kent - roasted hazelnuts

Krobus - pumpkin, void egg, wild horseradish

Lewis - hot pepper

Linus - yam

Marnie - pink cake

Pam - pale ale, parsnip

Pierre - fried calamari

Robin - goat cheese, peach, spaghetti

Sandy - crocus, daffodil, sweet pea

Vincent - grape

Willy - mead, pumpkin