'Interesting' to me generally means 1) strategic choices, and 2) a chance to meddle in the affairs of other players.
My first idea: add an option that instead of playing a card on yourself, you can play it on an opponent to send them backward.
But that seems too simple--it's too easy to hose other people. Perhaps there should be some blocking mechanic, something like 'if you choose to block with your card, no one else can play cards on you for a turn, but you only move forward one space.'
These are only tentative steps--I don't think that I would even playtest them yet.
I think a really good game based on Candy Land ought to exploit the fact that cards have different values to different players. A green card, for example, might move one player forward one space, and another player forward two spaces. And a 'jump to this space' is good if it puts you ahead and bad if it puts you back.
Hmm... consider this variation for 3 or more players:
You have a hand of five cards or so. On your turn, you play a card, which affects you and the player on your left. You then pass a card to the player on your right.
This might be interesting enough to try playtesting... there are difficult choices between helping yourself and not helping your neighbor.