Banging Lucy and beating up Carlo the first time.

Seriously, his wanting to get into the drug trade was actually a good idea, especially in the long run. It was the way that he blurted it out that was stupid, but his wanting to get into it would have actually been good for the family. In truth, Vito was being somewhat ignorant and old fashioned in not wanting to get involved in the drug trade.

So in reality, yes, Sonny did have a good idea when he told Vito that there was a lot of money in that stuff and then having an interest in it when he saw that Tattaglia's were going to guarentee the Corleone's investment.

He just opened his mouth at the wrong time.