You can shake things up in your house with little effort. I moved the crates into the dining room for a bit. I probably don't do it as often as I should-the purpose is really just to get the dogs used to being crated somewhere else besides the bedroom. This is useful if you are ever going to travel with your dog.
Of course, traveling with your dog is great stimulation and training too. Greta will be riding shotgun next month when I go to Houston. I'm still up in the air as to whether or not to take a second. Both Cody and Prissy could benefit from sharing Greta's love of the car.
Another good shake up is to train your dog while YOU sit down-useful if you ever end up in a wheelchair or need to shout a command from a park bench.
The thing is, those little changes provide mental stimulation for your dog, with very little effort on your part. Training is more about every day things than a once a week class. (Not knocking the classes at all-they are essential-even trainers take classes).