I don't have kids, but based on my personal experience, I think that just encouraging girls and boys to play together would help.
I have a brother, so he just naturally wanted me to include me in the things he did just so he could have someone to play with.
And outside the home, when I was growing up, I played in the playground with other boys and girls because we were all there and we just wanted to play together, so we just did the same things together.
I remember playing superheros in nursery school because everyone played superheroes.
In school, girls and boys had gym together in elementary school and junior high. We didn't get segregated until high school.
I have a niece and nephew, and DH always goes out of his way to make sure that he gets them individual, interesting gifts that aren't gender stereotypical. My niece, who is 9 now, said that out of all the presents she gets, the ones from him are the best. (The last thing he got her was a wind up radio.)
The above being said, I think a lot of it has to do with paying attention to a child as an individual and not making assumptions about what a boy or a girl would like.