Like many people have said, regular exercise is a great way to stimulate the brain.
Go for a walk in a park or forest if you live near one. Not a run, mind you, as the point is to observe and be attentive. Take everything in and learn to see things as they are; the texture, shape, colours...you'll find a renewed enthusiasm for life and learning when you can appreciate even the mundane.
Of course, exercise that gets you moving is great too. Particularly exercise that doesn't include repetitive, grinding motions. Instead of running on a treadmill, go run someplace you've never been before.
What you eat also affects your mental acuity. There's plenty of information on nutrition available out there so I don't need to go into much detail. Avoid sugar, caffeine and other stimulants that cause dependency, crashes, and that tax your body.
A lot of people are mentioning chess and other games that promote lateral thinking which is great. Just thought I'd bring up some things you can do with your body that overall will increase your mental agility, health, and mood, which in turn means you'll feel less lazy, unmotivated, and bored. Reading, meditating, and actively NOT watching TV work great too.