If you like watching TV, you can do that while mindlessly
clicking/tapping the interact button on one of the endless
clicklists. The Amulet doubles your income, so go for that as soon
as you can. Putting things on Auctions, notably Retro Pokemon from
Concentration, for a few hours can also help. A slightly less
active way I know is the berry garden, where you can do tasks from
the bulletin board.