What Breed of Dog would be the best to adopt, i have a 14 month old cairn terrier?

as you probably know cairn terrier's (or at least mine) is very hyper and loves to run around. i have a male and he has been fixed. i own a home with a fenced in back yard for him to run around in but i also work from 8 am to 6 pm and he is frequently there alone all day (depending on if my fiancee can come over to let him out) so i'm just looking for thoughts about what breed i would like another smaller dog. Thanks for your help!!!