Retirement is a phase of life where people don't have to worry about the work timings, deadlines to meet, earnings, and many other things they had to deal with when they were working. Your dad is one of those people who worked for several years to give you a good life.
You can get anything related to his favorite sport. Season tickets to his favorite sports team could be a unique retirement gift for your dad. Give him t-shirts and some sports gear to boost up his mood and enthusiasm.
You may also go for some personalized gifts, like your dad's favorite photograph in a beautiful frame, or a watch with his retirement date inscribed at the back, or a set of beer mugs with his name engraved on them.
These are a few gift ideas that may prove useful while choosing a retirement gift for your dad. You can also think of something innovative and creative according to your dad's likes and dislikes.