Consider postponing retirement. Work is a great way to make new friends and join new social groups.
Join clubs and associations that focus on your hobbies. For instance, if you like boating, call up a local boating club. If you'd like to take up pistol shooting, join the local pistol club.
Volunteer. Volunteering is a great way to meet active "doers," who often are engaged in the community, working toward a cause or working toward a goal.
Keep your look stylish and current. If you look attractive and feel good about yourself, you will radiate positive energy that is more likely to attract others into your life.
Throw a party. This is a great way to meet people who enjoy parties, who are generally fun-loving. As well, this is a great way to get invited to others' parties -- and those throwing their own parties generally are either active, fun-loving or both.