I'll stick with baseball because that's my favorite. Most cities across the country have remodeled or bulldozed their old stadiums and most of them are now spectacles beyond belief. Our local stadium AT&T Park home of the San Francisco Giants is a true modern marvel. You can enjoy the food, the coke bottle slide, views of the bay, the garlic fries, and of course the Giants-one of the most fun team in the major leagues.
But the Giants aren't alone I'm pretty sure all of the stadiums in the country- with the possible sad exception of the Oakland A's- are a venerable amusement park of fun and America's Favorite Past Time for people of all ages.
Baseball not your thing? Football is a lot of fun too. Of course in some cities you may freeze but it's fun and the tail gate parties are great.
It doesn't matter which major sport you choose you should definitely buy tickets. Sporting events are expensive but there are always deals if you are flexible and not looking to catch the visiting Yankees.
One suggestion-root for the home team-as much as I'm a New York loyalist, I've long given up on wearing the visitors jersey. It just isn't fun-be part of the community-catch a ball, have a hot dog and yes a beer-buy a hat or a foam finger or a panda hat, root, cheer, wave, do the 7th inning stretch-have a great time.
For the sports fan on your list I humbly suggest tickets to see your home team play something.