Maybe you could give them a garden plot where they can plant things they've found and grow their own for the rest of the campaign, but you can't ever buy a potted plant for your garden, you can only get it through foraging. So, it would be rewarding to find a new plant to add to the garden, but not tedious to have to keep resupplying every type of resource.
You could then set some rules around how frequently the plants can be harvested and implement a mix of time between harvests, plant knowledge, and a random component to determine how much they get each time they go back to the garden to resupply.
You might incorporate some random events where they get new items to help improve their garden. Maybe some special fertilizer, a magic sprinkler, extra-sharp garden shears, a magic music box, a singing sunflower
If they invest the time into foraging, they could potentially have a full garden by the end of the campaign that keeps them well stocked. Along the way, maybe they'll run into someone they can hire to help them tend to the garden and sell any extra resources. Maybe a guard dog to protect the garden from thieves or wildlife.
I'm thinking the foraging should be reasonably quick, so they feel worthwhile, while the events for special items might be a little more involved, but have a meaningful impact on their garden. I'm not sure how elaborate your setup is, but it would be really cool to be able to show them their garden growing and improving and maybe even have some cool decorations available for sale in the town(s) they visit (garden gnome, wind chime, scarecrow, etc.).