1. Add politics to the game (optionally, or maybe as an alternative Chance or Community Chest). Pay a lobbyist or contribute to a politician to put deed restrictions or zoning restrictions on another player's property, cash out another player's property through eminent domain, add rent controls, break up a monopoly and auction the pieces to the highest bidder, etc.
2. Make hotel and house pieces stackable so that players can create skyscrapers.
3. Add an optional dummy player represented by a monkey, who buys everything he can. See if you can beat the monkey.

The game is classic. Shouldn't really change things too much.

Peter