Bird watchers keep detailed field notes and are very proud of their 'life lists', accounting for every unique bird species they've ever seen. I am not a bird watcher (or 'birder' as they are sometimes called). I do have an old Peterson's field guide that has been used perhaps a half dozen times in 25 years. Instead, I collect ballparks.
In approximate chronological order (major league teams, including spring training and exhibition games):
1. Memorial Stadium, Baltimore
2. Comiskey Park (old Comiskey), Chicago
3. Wrigley Field, Chicago
4. Candlestick Park, San Francisco
5. County Stadium, Milwaukee
6. Metropolitan Stadium, Bloomington, Minnesota
7. Tinker Field, Orlando, FL (Twins spring training)
8. Oakland Coliseum, California
9. Hubert H. Humphrey Metrodome, Minneapolis
10. Comiskey Park (a.k.a US Cellular), Chicago
11. Doubleday Field, Cooperstown, NY
12. Miller Park, Milwaukee
13. Camden Yards, Baltimore
14. Fenway Park, Boston
15. Yankee Stadium, New York
16. Citizens Bank Park, Philadelphia
17. Angels Stadium, Anaheim
---- updated Sept. 2009 ---
18. That new ballpark named for some insurance conglomerate in Cincinnati (nice park, bad name)
19. Dodger Stadium, Los Angeles
20. Kauffman Stadium, Kansas City
---- updated April 2010 ----
21. Target Field, Minneapolis
There have also been dozens of minor league, independent league, high school, college, and amatuer games in various places. Each comes with a story. Cape Cod league games in the little resort town of Orleans were extremely pleasant. Stumbling upon an alumni versus student game in Hilo, Hawaii was fun. Going to the annual Labor Day weekend Minnesota amatuer (a.k.a. town ball) tournament was a slice of Americana. Meeting Wally the Beer Man at a Miesville Mudhens game -- priceless.
My grandmother would listen with great intensity and joy to White Sox radio broadcasts, so baseball was a big part of the soundtrack of my life as a kid, and it is reassuring to have the game, essentially unchanged (except for the damnable DH rule and artficial turf), to enjoy as an old dude.
My father will be 90 in a few months. His two most enjoyable outings recently have been to a vintage air show (he was a pilot in WWII) and to a minor league game in Indianapolis. He and I and many others use these memories to keep us warm in the winter.