The simple truth is you're responsible for your happiness in retirement. Don't expect someone to hand it to you on a plate. Assuming you can live within your means, then this generally relates to filling your time in a manner which makes you happy. The issues with this are:
1 - You're conditioned by a lifetime of working to being productive, it's a mindset thing which you need to get out of or you may as well have carried on working. If it makes you happy you shouldn't really give two fucks as to whether it's productive or not.
2 - You have to be inquisitive or creative enough to find that which makes you happy. It's not necessarily easy
3 - It's unlikely that one activity will tick all your boxes, you need to keep both mind and body engaged
I've been retired almost 3 years, my time is filled with two physical activities and 1 mental activity. I have room for more but as yet haven't found anything else which makes me happy. Still have a shortlist to try out though :)
In a nutshell, retirement, like life, is largely what you make it