Happiest day in aviation for me was the 2nd-last day of my 2008 19-day west coast adventure in the club's old 182.
On the trip, I had seen numerous beautiful sights from the air: Mount Rushmore, Crazy Horse, Devil's Tower, Mount St. Helens (and lots of others up there), Crater Lake, Grand Canyon, etc. I had taken a mountain flying course and landed at some beautiful backcountry strips. I'd landed on "One Six Right" and at the lowest airport in the US, Furnace Creek. I'd spent time with my family in Sunriver, OR and some great friends on the west coast. My friend Sheri (also a pilot) had joined me for the Oregon to Colorado part and this was her last day with me, as well as my last day of seeing beautiful sights before flying home over the relatively boring terrain of eastern Colorado, Nebraska, Iowa, and southwestern Wisconsin.
On that day, we took off from Bryce Canyon, made one more loop over the beautiful "hoodoos" there, and headed up into Utah. We flew over the San Rafael Swell, a geographic feature I'd admired greatly from the ground, and the rest of the stark, barren beauty of Utah. We flew into Colorado, where the vegetation is much more prevalent, past Aspen and Mount Elbert and the other fourteeners, and landed at Leadville (the highest public airport in ConUS).
We were on the last leg, from Leadville up over the Dillon Reservoir, through Loveland Pass, following I-70 down toward Denver and watching the setting sun create some amazing sky art on the side of a giant thunderhead south of us. The trip was coming to a close for the most part, sans that last day of flatland flying, and I just felt giddy. It had been an excellent learning experience, a lot of fun, and I got to see an awful lot of the most beautiful sights GA in the US has to offer. I've planned a thousand fantasy flights I haven't taken, and I love to plan flights, but to get to execute one with so many bucket-list experiences and have it not only live up to the fantasy but completely blow it out of the water was immensely satisfying.
BTW, that was pre-iPad and pre-ForeFlight. I bought all the charts, plates, and A/FDs and did it all the old-fashioned way (well, there was a Garmin 430W on board for actually flying it, but...) All of that paper cost me $225 and filled two grocery bags!