Optimal for fun, given the PERMA model within Martin Seligman's happiness studies:

Should be a group activity Frequent goals, scores, or other sparks of dopamine. Or a feeling of constant flow. For males it can be positive to have women doing it, both motivationally and to diminish hierarchical male-only stress and bullying. You should probably not absolutely suck at it, after training, to avoid frustration.

I've found the most fun exercises to be, in this order:

Sex

Ecstatic Dance (oxytocin bonus points for touching)

Contact Improvisation (also a touch dance)

Ultimate Frisbee

Tennis

Dance Walking

Fighting waves in the ocean or bodyboarding them

Rollerblading.

Honorary status for Volley, Handball and Basket ball due to frequency of scoring, compared to others.

Not recommended for happiness alone would be: Lifting Running Speedbiking Competitive distance running Soccer Baseball (bad ratio of time doing stuff versus not, few hole in ones) Swimming (if you don't feel flow) Abs, push ups, all that stuff that you only do for instrumental reasons and for the endorphin, which all other exercise also provides. Yoga (if you don't flow or connect with the universe when doing it)