Play Tug of War
A word of caution: if you haven’t trained your dog or he hasn’t himself learned the basic bite inhibition; he’s probably going to have a hard time sticking to the rules of the game. So make sure you teach your puppy to not bite before you go grab yourself a tug toy.
Tug is an excellent game to exercise your dog indoors. It provides physical as well as mental stimulation for your dog, it’s fun, and it's easy! If you don’t happen to have invested in a store-bought tug toy, you can always DIY a tug toy. Use an old T-shirt or a rope that’s lying around your house.
Happy Tugging!
Swim with your dog
A lot of dogs like retrievers are natural swimmers and love some swim time every once in a while. It’s also ideal for dogs who have mobility issues like arthritis. Swimming, being a non-weight bearing exercise, is an excellent way for your dog to build some muscle mass.
If you don’t have a backyard pool, see if you can take your canine buddy swimming to a public pool or a lake. Swimming can become an exciting way to spend your weekend with your dog in the pool. However, If your dog is not a natural swimmer, be sure to take it slow.
Let him meet with a variety of people and dogs
Expose your dog to a variety of dogs, children, men, and women so he can get comfortable in socializing. If your dog is fond of hanging out with one person, it can affect his social life. It is great to crucify his social calendar and pay attention to all his greets and meets.
If your dog acts strange and scared, do not push him to do better or make a great deal out of it. One way to encourage him to socialize is to give him treats whenever he meets a new person.
Have him sprint up and down the stairs
Use the stairs at your home to tire out an energetic, active dog. It also benefits the dog because climbing the stairs requires your dog to use different muscles than those used during a walk. Plus, the elevation also adds another layer of difficulty.
To use stairs for exercising your pupper, climb up the stairs and from up there, toss a toy down to the dog. As he excitedly rushes to grab the toy, call him up to you with the toy, just like you’d do while playing fetch. A few such sprints should do the job.
Be timely
3-12 weeks of age is the best time to socialize your puppy. It is a crucial time window in which you have to teach him to socialize or to poop at a designated area. Typically, new puppies in this age bracket are exposed to unfamiliar clothing (such as hoodies, jackets, hats, sunglasses), unfamiliar people (your aunt, nephews, friends), beaches, parks, water bodies, other dogs, and body handling.
If your dog is beyond 18 weeks, it might be a little challenging for you to socialize with him. However, you can use all the tricks that people follow to socialize rescue dogs.
Put him in dog classes
A boot camp specifically for dogs provides endless social and interactive activities at affordable prices. It is a great place to meet and greet new dogs and people. Moreover, the environment of the dog kindergarten is controlled and safe. So, no harm can get to your dog in any way.
Similarly, you can hire a dog trainer to help him socialize outside of your home. Dog trainers are professionally qualified to handle absurd and strange dog behaviors. 
Socialize your puppy