Use the number 75 and the guest of honor's interests as inspiration for 75th birthday party favors. Bake and decorate a batch of sugar cookies cut into 7s and 5s, and send each guest home with a pair. If the guest of honor loves golfing or travel, hand out golf balls printed with 75s or pocket-size travel guides to the local area.
One touching way to honor someone's 75th birthday is to make a donation to his favorite charity. Then fill small bags with three quarters each, and attach a small note that reads, "Buy something sweet with this 75 cents! In lieu of larger favors, a gift has been made to X charity in honor of Chris's 75th!"
Edible treats always make sweet favors. Paint 75s on mason jars, and fill bowls with different kinds of candy. Invite each guest to fill a jar to take home. Another option is to buy gourmet chocolate bars and wrap them with new paper labels printed with a birthday message or the date.
Make a playlist of significant music, and give every guest a copy. Include the No. 1 hit song from each of the previous 75 years, or pick one song from every five-year period (for a grand total of 15) so guests can hear the way music has evolved over the guest of honor's lifetime.