Maintaining a healthy cardiovascular system benefits your brain, protecting it from disease. So reports the UC Berkeley Wellness Letter in their May 2009 issue (http://www.berkeleywellness.com -- this is an awesome newsletter, btw; I've been a subscriber since the late '80s and their format and quality has always been consistently high). Here's a quote from the article:

"Research has consistently found that staying physically active is a key to preserving brain function. Studies have shown that older people who get regular exercise are less likely to decline mentally and/or develop dementia. Aerobic exercise such as running or cycling seems especially beneficial, but any activity can help, including strength training and ballroom dancing (these two were the focus of recent studies). Exercise probably benefits the brain just as it helps the cardiovascular system—by lowering blood pressure, improving blood flow, controlling weight, and improving cholesterol levels and blood sugar. In addition, brain scans show that aerobic exercise can actually improve brain activity and produce new brain cells and connections between them."

Speaking personally, I find that engaging in moderate aerobic activity tends to stimulate my thinking process, promoting creativity and problem solving. Part of this is probably due to the change of scenery and getting outside in the fresh air. But I think most of it is due to turning the crank on my heart rate.