Not only you have to walk and feed your dog, but you have to train it! You can train your dog a simple sit and roll over, or a difficult backflip and a spectacular handstand,uh, pawstand. Or you can practice disc throwing or agility at a gym. Also show off what your dog knows! Enter contests and show everyone what you and your dog can accomplish:

   Obedience Trials: You must teach your dog certain tricks in order to win at obedience. Not only the judges will score on performance, but the cleanliness of your dog. Go to the care category of your supplies and read the obedience trials book for more info.

   Agility Trails: To Enter in agility trial competitions, you must train your dog at a gym. To get to the gym, take your dog for a walk. Once you reach the gym, practice going through obstacles. Also, keep practicing after contests  to become a real pro!

   Disc Competitions: To practice, make sure you have a flying disc with you when you take your dog for a walk to the park. Go to an empty park to train, so your dog doesn't get distracted by other people's dogs. For more info on contests, read the Agility Trials Competitions Book, Disc Competitions Book, or the Obedience Trials Book.

Nintendogs Tricks: Best in Show!
Tip: Confused what to do in a contest, or how to take care for your dog? Read the books in the care supplies!!
 
 Screenshots & Videos:
Agility Trials
Disc Competitions
Tip: If you really want to win competitions, practice, practice, practice!!!
Good luck and have fun!!!