Mental stimulation and enrichment is vital for dogs (and cats) to lead a healthy and happy life. Many dogs stay at home during the day while their people are working or out at various activities. All that alone time can lead to problem behaviors stemming from boredom. The following are products I personally use and recommend, and where possible I have provided a link to these products for easier purchasing. Never leave your dog unattended without first watching him/her to see how they will react and whether they will destroy them in a dangerous way. Some may work better than others for your dog.
DIY – Low Cost
Free or low cost items to make yourself
Slow feeders and food puzzle ideas to work on mental stimulation
Toys and More
Interactive and other fun enrichment items