Interests/activities/hobbies on a resume?

Should you write interests/activities/hobbies on a resume?  
Not if you don’t feel comfortable doing it. However, it does add to the picture of who you are and can increase the likelihood of securing an interview.
Interested in sports? It can mean you want to be fit, or that you like to win, or you enjoy playing in a team, or you do it because you like the socializing afterwards. Maybe it got you a scholarship at college. All of this is good, so why not write that you like baseball/tennis/water polo/running/synchronized swimming?
All other interests are relevant, too. Charity work, church activities, hanging out with your family and friends, reading, music, crochet…all good. I had a candidate say, “I go sailing for the socializing afterwards. Is that a terrible thing to admit?” Absolutely not. Another candidate was a pharmacist with an entrepreneurial streak. He had been the Under-16 National Greek Ski-ing Champion. So THAT was an early clue, right there on his resume, that he would perhaps be a driven personality who wanted to succeed. As indeed he did.
Hobbies round out the picture of who you are. You may have interests that are similar to your job e.g. you’re a staff accountant who teaches mathematics to under-privileged children. You may have an interest totally dissimilar to your job e.g. you’re a trader who grows orchids in your spare time. All of it makes you YOU. Who you are can be just as important to a company as your being able to do mailmerge.