They can't see in your cabinets unless they have a key, they can't get into your sdb's even with a key unless you set a price. I don't think they can take your crops, but they could poison a tree - that would be very unusual though and pretty rude. You can kick someone out.
Harvest/Plant crops: Keyholders
Access items dropped in house: Anyone
Access items "placed" in decoration mode: Only home owners (this has been bugged in the past though)
Cabinets: Keyholders
Poisoning trees: Keyholders
SDBs: Only the keyholder unless a price is set
Here is what I know from my researches:
guests can't take anything except it is drop on the floor. Everything else like an item above a stool, above a sofa, inside sdbs, inside cabinet they can't take. Resources work the same like in homestreet.
keyholders can do everything just like owner does except taking stuffs inside sdbs. Although this probably will change.