(Note that unlock numbers may not be completely correct. They should at least be upper bounds.)

Spooky Tree
At 750 trees, one of them will get a face. The face can be fed apples from the witches' caudron to get a bonus.

Jack O Lantern
At 100 Jack O Lanterns a hammer will randomly appear. Hit a Jack O Lantern with it, and it will break. Every 3 hammers will give you a bonus.

Tombstone
I do not know of any actions you can do to these.

Ghost
At 250 ghosts, ghosts will start rising from tombs. A ghost trap will also randomly appear. Pick up the trap, and click on a ghost to trap it. The color of the ghost shows what bonus you will get. White ghosts do not give a bonus.

Spider
I do not know of any actions you can do to these.

Bat
At 50 bats, bats will randomly fly from the house to the outside of the screen. Clicking on them will give a small money bonus.

Black Cat
I do not know of any actions you can do to these.

Witch
At 50 witches, a witch will stand on the front lawn, and can be hit with a bucket of water to get a bonus spin.

At 200 witches, a cauldron will be added. You can find eyes and apples in it. Feed them to zombies and trees (with faces) respectively.

Zombie
At 25 zombies, zombies will start walking across the screen. You can feed them eyes from the witches' cauldron to get a bonus.

At 100 zombies, the hands reaching through the dirt can leave a bucket behind. This can be dumped on a witch to get a bonus spin.

Mad Scientist
After Mad Scientists have been unlocked, a random window may flash red. Click it. This will start a game of "Simple Simon". Click on the windows in the order given. (If you miss the pattern, wait; it will repeat.) Rewards are spins and gems, and are higher the longer you play. Low-level rewards:
1-4: no reward
5-9: 1 spin
10-14: 2 spins
15-19: 3 spins
20-24: 4 spins
25-?: 1 gem
(You get one point for each correctly picked window.)

This is a bit subjective, but hopefully still useful. It's ordered roughly in the order you'll want to buy these.


Auto collect: This lets you hold more than one random drop at a time.
Auto collect slots: get a couple of these.
Hidden spawn 30 s faster: get a couple of these as well.
(These can be considered an investment. They let you collect more random drops, which include gems.)

Hammer spawn 30 s faster
More colored ghosts
Trap Spawn 30 s faster
Soup Spawns +1
Bucket spawn 60 s faster
(These are also an investment. Here, just get the first level or two, and only once you've actually seen the object in question. Otherwise, it won't actually matter.)

Bonus speed length x2
Bonus income length x2
(There's two of each of these, you want both. These let the speed and income stick around longer, and so give you more money.)

Spinner: replace click bonus with time warp
(This makes using the spinner almost self-sustaining, and gives you a bunch of money as well.)

Zombie Income x2
Scientist Income x2
(Again, two of each of these. These two give you the majority of your money in the game, so doubling their output is really good.)

Spinner: Replace income bonus with money
Spinner: Replace speed bonus with +2 top item
(If you have all 3 basic spinner upgrades, the dual bonuses also disappear.)

Show all $/s +10%
Show Scientist $/s +10%
(10% bonus income. Neat.)

Front door key. (You'll need it eventually. Get it from the tool menu, not the scene menu. It's cheaper there for some reason.)

Anything below here is more "nice to have".
Spinner: add large, hard to get prize. (Odds do not match the visual size of the slice. It's closer to 1% or less. Still, you'll hit it a few times.)
Double Spirit Bonus (It's better than the auto-clicker, but it's expensive.)
Auto Clicker + upgrades. (I just don't think it does enough, and it's expensive. You need the auto clicker upgrade, a few double auto clicker speed upgrades, both bonus bar fills faster upgrades, and it's still not great. One thing it's good for is the achievement where you click on the house a bunch of times.)

It is possible for using the spin to win wheel to be nearly free. Start by getting the 3 main spinner upgrades, a few auto-collect slots and at least 100 spins.

Now, remember that gems can be converted into spins at just about a 1:1 ratio. Take a look at the following wheel:

Ignoring the bottom slot, (it has about a 1% chance to get hit) we have a 1 hour skip (expected return, let's say 3 gems.), a +1 gem, +2 gem, +3 gem, and four slots we don't care about. If we spun the wheel we would have an average payout of (3 + 1 + 2 + 3)/8 gems = 9/8ths of a gem, or around 1.4 spins.

Now, not all spins will have an average payout greater than 1. The trick is the spin x10 button. If the average payout is 1 or more, you spin x10. Otherwise, you just spin normally. In order to do this fast, I recommend just counting the number of spaces that have +spins, +gems, or a time skip, and using the x10 spin if there's at least 4. (Note that a ten hour time skip does not have ten times the reward of a one hour time skip, as there's only so much space to store bonus items. I don't think this matters too much.) Once you start running low on spins, you can buy more with the gems that dropped.