If you really want those pushups to do some good, once you can do 20 pretty easily, you should make them more challenging. Bring you hands closer together, put your feet up on something, or come down on one arm more than the other.
Or try Hindu pushups;
They're a smoker. Or tie it all together with some classic burpees. Get the full body workout.
Fingertip pushup also helps to strengthen your grip. Putting your thumbs and pointer fingers together in a diamond shape also increases the difficulty. Setting your legs up on the couch or raised positions makes them a tad harder. Also I personally find keeping your elbows back against your body instead of clear out changing how the pushup feels and increases difficulty. It is now weird for me to out my arms out! To make it extra fun do your 19 pushup then go down slowly, hold until you can't hold no more then go up and call it good!
I switch it up a bit. Sometimes I put my hands up higher, sometimes lower, closer together, or farther apart. Thanks for the suggestions. I'm not looking for serious strength or muscle building, so much as maintaining a healthy level. They've definitely become much easier since I started, which suggests to me that they keep me at a higher level of fitness.