I love playing "finding" games with my pup - they work on the same principle as expensive "dog puzzles" that you can buy, but they are cheap/free, and you can keep switching it up as the pup progresses.
For example - take some sturdy plastic cups (little flowerpots made of plastic work well - you don't want anything that will break easily) and a ball (or treat/kibble if you want). Have the dog sit, and watch you as you put the ball under the cup (use BIG hand movements to start) - then move back a little, and give the ok - see if the dog can flip over the cup to find the toy. You can progress to using multiple cups but one ball, moving the cups around, or playing "hide and go seek" style - hiding the ball under or in a variety of objects, and having the dog find it. Kibble is usually easier to find because of smell, whereas toys can be a little more difficult.
Make sure that what you are doing fits with your dog's capabilities and attention span - some pups get bored of a Kong if they can't figure it out fast enough, and then can get frustrated - smelling a treat but not being able to get it. Work with your dog's personality and level of attention to a game.
Up your training of the more "fun" commands - things like rolling over, playing dead - stuff that you would never use in the outside world, but that can be fun and mentally stimulating to learn.
And make sure that you are doing your games at a time when your dog has been fed and had a little exercise - it's just frustrating for everyone to try and train or play games if your pup is bursting with energy and wants a walk, or is distracted and hungry...or on the other end of the spectrum - exhausted from play and just wants to sleep.