I think that I tend to "enjoy" rather than "have fun". There are some things that are fun, but I seem to do them less and less.
To me, something that is "fun" tends to involve a higher than normal amount of activity and stimulation of multiple senses.
For example, when I was in high school we had some of the old round, concave signs advertising cokes such as the one in the picture here:
After a snow, we would take one of the signs out to a nearby pasture, tie a rope to a pickup, and one person would sit in the sign and hold on to the rope while the others drove or rode along in the pickup with the driver. Going along at 40 miles per hour and turning results in a far higher speed for the person sitting on the sign and a great deal of force -- it can be really difficult to hold on. Running someone through a barbed wire fence or over the side of a canyon was off limits.
That was fun.
Other kids would ride on upside down car hoods pulled by a pickup. In comparison to the coke signs, car hoods are rather unexciting.
Racing someone down a dry, sandy creek bed on horseback was fun. I had a horse fall on top of me once while in a creek bed miles from the nearest house and that was fun since neither me nor the horse were injured.
Sometimes even work can be fun such as driving cattle from one pasture three or four miles to another pasture can be fun.
When I was in elementary school, I would climb up as high as I could in the big tree next to the house, grab firmly onto the branches, and then sway back and forth over and over. That was great fun. Fortunately, I never made the mistake of doing that when adults could see me do it.