I think one of the most important things to do if you're 50+ and single is to make sure you get out of the house and do something. There really isn't any excuse why you can't. I know.... a lot of people say they don't have much money and can't afford to do anything, but there are lots of fun things you can do without a whole lot of cash.
Hiking: Get out of the city/town and go find a friend to go on a nature hike. Most provinces and States have nature trails. It's sure to lift your spirits.
Biking: I often bike in the summer time here in Nova Scotia. There are a lot of back roads to explore. And by biking, I don't mean an expensive motor-bike or ATV. Just a regular bike. Mine is a 18-speed mountain bike, which is great for some of the back roads I take it on. It's amazing some of the things you can see and do while on a bike.
One time on a woods trail my daughter and I came around a steep bend on a woods road to see a coyote pup playing in the middle of the road. The mother wasn't far away, but we didn't bother them, and they didn't bother us. We also stopped on the ride to pick some raspberries and wild strawberries that were growing wild beside the path we were on.
Fishing: It doesn't cost much to go fishing. Pack a lunch and make it a day trip.
Bowling: There are lots of places if you're single and 55 and over where you can join bowling teams. Even if you have to go by yourself, you're sure to make some new friends.
Card parties: Well this one certainly doesn't need any explanation.
Start a business: Hey! If you're passionate about something and need to earn some extra cash, there isn't any reason why you can't start a little business. This one will certainly take up your free time. Believe me! I know this to be true!!
Volunteer: I'm sure you can find some places where you live that are always looking for volunteers. What a great way to meet people. And you can feel good about your day when it's over because you have helped out.
Take a course: A lot of the community collages these days have courses and programs for older people. Check out what is available. It's another great way to meet people and keep your mind sharp.
Weekend road trips: In the Summer time it's easy enough to throw a pup tent in the trunk of your car and just go for a cruise. Most provincial/state campground don't charge much for pitching a tent. You can take your meals mostly prepared with you, or buy stuff along the way that isn't overly expensive.
Throw a BBQ party: And make sure some of the friends you invite over are musicians. Nothing like a good steak, a little wine, and a lot of dancing.
There's always things you can do. The beauty about being single, is you can make plans without having to pass it by anyone else. You can do exactly what you want, when you want to. I think it's a good thing.