My dad was an ME who planned to retire at 65, but when he made the announcement, his employer made him a sweet offer to stick around that included a big pay bump and reporting changes to stick around another 2 years. At 67, he retired, but then got an even better job offer from a competitor. He worked another 2 years, got fed up, retired again. Got an even sweeter deal with more pay and freedom, worked another 2 years at that job, before finally retiring in his early 70s.
I think for him, one of the hardest parts of retirement was the loss of social interaction. While an engineer, he was also in a sales role and he's a people person.
Now he spends his time with grandkids, house projects, and international travel with my mom a couple of times a year.