Everyone has had some great suggestions. One other thing my DH and I have done is for him to ride on ahead for 3-4 minutes, then turn around and ride back to me and/or past me. He can ride his speed, I can ride mine, and we're always fairly close together. He likes this a little better than stopping and waiting for me because he can keep on riding. I like it because I'm not looking at him in the distance all the time and yet I'm not riding totally alone.

Now that I'm running and swimming, and riding a little less, he does hard rides alone, and easier rides when he's with me.