Your brain is not only like a muscle. It's also like a 10 year old. It could have started out curious and as you nourish that curiosity and savor the habit of learning more, of seeing relationships among things, of solving problems, you tend to improve the cognitive aspect of your brain.
Problem solving is a good way to keep your brain sharp. Once you get familiar with how your biases clog and impede your way of seeing things, and get some training on problem solving techniques, not only will your stress level go down, you will start to feel empowered. Most people try to avoid problems, but if you're curious on how to generate solutions to things and to try them out, then problems become opportunities to make things better. The more empowered and capable you feel about your problem solving skills, the better you feel about yourself, the more you become capable of dealing with stress. Consequently, the more capable your observational and analytic skills become, the healthier your brain gets.
So it's recommended to start researching on problem solving techniques, and see how the many strategies may apply to your various domestic, academic, and work scenarios.
On the other hand, your brain is like a muscle. It atrophies or welts when you don't use it. Like a plant, It doesn't grow well when it doesn't receive the right nutrients. Like a dog underfed and eating trash, it limps and gets sick easily.
There is also a social and self-affirming positive outlook about trying to better your brain. It involves spending time enjoying the company of your friends and family, when you can be yourself, and just let it all hang. There's something spiritually relieving about being yourself, and around the people you're comfortable with, and whose company you enjoy. This kind of rest is good for your brain, too.
Get some sleep, a lot of sleep. The brain works better when your synapses are healthily firing and your subconscious has had time to sort itself out. This means rest physically and mentally. The more time away from a problem you get the more your mind has time to work on it, subconsciously. It's always better to approach work problems after a long night's sleep.
Work out, too, on a regular basis. The more your heart gets pumped and often the more blood circulates throughout your body, and to your brain, bringing fresh supplies of oxygen and nutrients to it. Also, working out brings about increased levels of release of endorphins that make you feel good. It's no wonder some people become addicted to exercise. Your brain will like it, too. These are just some of the ways to better your brain.