The only activities that I encouraged my son to get involved with to impress college coaches, were activities that I already wanted him doing (and college baseball was just an extra incentive). Those were things like getting good grades, working hard, good sportsmanship and being a good citizen.
With recruiting, I think it’s a mistake to try to represent yourself as something that you aren’t. The courtship analogy works well here.
If your kid is a diamond rat and wants to compete year-round, then nurture that. Don’t make him play a sport that he doesn’t like just for his resume.
When baseball coaches talk about multi-sport athletes, it’s usually a guy like Michael Vick or Carl Crawford. Guys who are just freakishly athletic and can excel at any sport. They know that a kid who starts at QB, point guard, and short stop on his high school team is probably going to be a stud on the diamond. Part of the appeal may be that the kid loves to compete, and is coachable, but mostly it’s the athleticism.
My son lettered in 3 sports this year. He plays the "other" sports because he loves to compete, he loves the team comradery, and his school has a proud athletics history and there is a definite sense of obligation to do your part. My kid is not a stud athlete, and I don’t think his multi-sport status made any difference in his recruitment.