I am a beginning runner, and I would love to be a really great Olympic runner. (not really happening except in my dreams 
).
Marathon.
Mountain biking summer games
Snowboard winter games
Gymnastics. I never got farther than doing a somersault on the balance beam, or transferring from one to the other on the uneven bars. I have such admiration for those young women.
Realistically, rowing.
Equestrian. I just finished looking up Mark Todd on Wiki because I remember his dominance in the sport from my late teen years, and he’s still competing at age 60 - currently in 4th - on a course that took out about a third of the competitors (most from refusals, not falls).
I rode back in my youth - even doing a (tame) cross country course or two. We still own two pony broodmares raising ponies that compete (once we sell them). A few get to National Pony Finals, but what we do can’t even begin to compare. He’s my hero/idol when it comes to sports. His cross country ride yesterday was almost perfect, and given his age, wow!
Hubby would choose sailing.
I admire the white water competitors in canoe/kayak, but there’s no way I could ever think of trying that one. My canoe/kayak experiences are calm water relaxations.
I think badminton would be fun - one of the few sports with mixed doubles - could compete with my spouse?
I think I’m going with archery. I remember years ago there was a kid from California in the Olympics and he use to practice by going over to the neighbors and shooting across the street, through his garage passage door and out to the target in his backyard. I thought it was so cool!
I’ll always been intrigued watching rowing. The camera angle makes it look so easy, though I know (and the closer camera views show) it’s much harder.
Badminton. It’s inside (air conditioning! No mosquitoes!) and I’m unlikely to hurt myself seriously :).
I have dreams of competing in dressage, if only to own one of those gorgeous horses. And there was a guy who competed in dressage on the Olympic level until he was in his 70’s, so there’s hope for me yet!
I think I would like to do either swimming (if I ever learned to really swim correctly) or sailing (I sailed on the collegiate version of the 470 class, the 420, not very successfully I might add:).
My adult self would probably pick running, since I enjoy it even though I am slow.
As a child I dreamed of being an Olympic gymnast (I was 7 when Nadia fever hit). I did gymnastics through 10th grade or so but my big flaw was being petrified of the balance beam. Loved floor and bars, even vault was okay but ugh–the beam.
I added ballet around 6th grade or so and was also on the community swim team–so then as a teenager I thought I would like to be a synchronized swimmer (though those nose plugs are so weird looking).
Sailing because its a passion. Though given the option, I would prefer to sail in the America’s Cup over the Olympics.
Gymnastics. I loved the spring bar. In junior high, I’d spend half my recesses on the bar doing pull-overs and flipping around just for fun. Not sure I’d have been particularly good at any of the other events, though.
I’ve been sailing since grade school and although I have a passion for it, I’m not sure I’d enjoy doing it competitively.
Sailing for me, too. It’s the only sport I ever competed in outside of high school PE class. No gymnastics - most are considered over the hill by 25. That would be sad.
Equestrian - specifically showjumping. My Ds rode and competed for many years and I loved watching them. And as long as you stay healthy and fit you can compete for many years - Beezie Madden is 52 and she is on the American team! Lucy Davis is also on the American team and rode horses here in Southern California at shows my girls were at when they were younger.
Hmmm.
Anything Basketball,Cycling,Rowing, or Wrestling related
I’d probably have to say BMX. It was in London & Beijing & looks fun.
I think Rugby is in this year, so you can add that to the list also
800 Meters. Most demanding event in track.
Gymnastics. I was the same age as Nadia the year she first earned the 10s. I loved how intent she was, even though the commentators were always grumbling she never smiled.
100 meters, without a doubt. I was the city champ in the 100 yard dash at the 6th grade all-city field day. When the Olympics came on after that big win, I was so excited … I ran around the block so many times that summer! Oh, I wanted it so badly. But alas, I was from a time when parents didn’t run out to get their kids coaches when we said we wanted to do something. 
I ran in middle and high school, won a few races in college intramural meets, and now my back limits me to the elliptical rather than the track. But I still dream of what could have been when I watch that 100 meter race!!
Seven-man rugby.
During the 1976 Summer Olympics I was an exchange student in Europe working on a farm all day long. I made a makeshift 100-meter course in a dirt road and ran sprints every day pretending to be in the Olympics. I bet a few of the local grain farmers and their horses thought I was nuts.