Just play through all the game's levels on one difficulty mode. If you don't want to be challenged by time limits, Easy Mode is what you should pick.
If you're on Normal or Hard, the pause screen offers a Level Guide for each level. You'll likely need to first unlock bonus choices to get three stars on the harder levels.

The game's 60 levels each have one hidden owl behind a tree. (There's a "Wanted" list for owls. Perhaps some pun akin to Outlaw – Owl.)
On mouseover, you see a "Owl" tooltip. The owl periodically jumps up behind the tree to make spotting it easier. You need to (and can) collect each level's owl only once.
If you miss an owl, don't fret. It's enough to start the level again, click the owl, and exit to "Map"!
Exiting does NOT persist your other progress, so don't try to cheese the achievements below that way.

In addition to the levels, there is a tiny shooting gallery minigame. You have to click anything that moves, namely robbers and ducks. Not shooting robbers makes them 'shoot you' for a point penalty. I don't think it's even possible to get less than three stars on Easy mode, but in any case, I suggest you double-click or even triple-click anything that moves as the hit detection on moving objects is kinda wonky.

Contrary to what it says, you don't need to "fully" (twice) upgrade houses for this. You'll get this anyway while grinding gardens.

This is where we grind. I had ~40 left to plant after finishing all levels, and after some trial and error, my recommendation is:
Level 26, with upgrades: cash income, food income, totem cost, worker speed. In around 6 minutes or a bit more, you'll get 4 gardens. Repeat (using the "replay" button after the level to save time) ad nauseam. You need to finish the levels. Exiting to map to save your gardens does not work.

Level 28 (8 minutes) also has 4 gardens. Levels 40 (7-8 minutes) and 47 (6 minutes) only have 3 gardens. 25, 27, 29, 30, 39, 49, 1B, 6B, 9B, 10B are some other garden levels if you'd like a little variety.