I am a Vet assistant and I own many pets, I have experience with all sorts of breeds, but I would suggest a smaller dog that is lower energy. Some of the best friendliest dogs I could suggest would be a Pug (Own one) she is my little shadow, not a super high energy dog and not too dismayed if you don't get out for a walk. A Corgi would be another of the same line of activity. A Higher energy dog, more the grab the leash on beg to be walked would be a Papillon (Own one),Maltese, West Highland Terrier, or Poodle/Min Pin. Once again with thee extreme energy your looking at a Jack Russel Terrier. I guess it would depend on how often you walk your Lab and remember that some of the smaller more energetic breeds will be more on the jealous side when they are not the center of your world! Check out the attached link and see what they would suggest!!
Adopting is a wonderful idea, I got my Papillon from Kijiji in the free pets! Just because they are not in a shelter doesn't mean they will not end up there if the owner can't find them a home first...