You can buy a pet, or see what they usually like, buy equipment (such as fishing equipment, golf equipment, blablabla, remember to buy one), or sign up for some classes for the elderly, if you know that your parents are particularly interested.
Retirement is an obstacle. If you are suddenly idle, you will feel at a loss and lack a sense of existence. For a while, your temper may get worse, especially when your parents used to hold high positions (don't ask me how I know my grandfather just retired). You should find something to help them divert their attention and greatly distract their energy.