My Clue Variant
This variant of Clue will make the game more challenging without making it less fun. There are two differences from the original: how you move and who may show you cards to disprove suggestions.
Throw away the dice, you won't need them. You can move your token anywhere you want, or stay in the same room for as long as you want. There is no restriction on moving. But you will want to be more careful about where you go, as you will see.
Take a moment to notice how the Clue board is laid out around the perimeter of the mansion. Closeness of two rooms is defined in terms of rooms around the perimeter. So the Hall is two rooms away from the Dining Room, and the Kitchen is three rooms away from both the Billiard Room and the Hall.
When you make a suggestion, only the closest players may show you a card, and each of the closest players must show a card if they can. For example, if you are in the Ballroom with one other player, only that player may show you a card. If you are alone in the Ballroom, with a player in the Conservatory and two players in the Kitchen, then those three players may show you cards. All of this is calculated after you make the suggestion, so if you suggest Miss Scarlet did it and one of the players is Miss Scarlet, then only that player and any other players already in the room may show you a card.
That's all there is to it. The resulting game is more challenging because you get less predictable and less complete information, which means you need to be more creative in your deductions. It is less boring (or at least not more boring) because you don't have to worry about getting stuck walking in the halls or across the board from where you need to be.
My brothers and I have played this way a few times with exciting game play to the end each time. Usually when we play normal Clue the game ends quickly, before anyone feels like they've had a chance to do any serious thinking.