How can you increase brain activity to delay or even prevent the onset of alzheimer's?

Different activities. Though i don't know of research, the idea is to do different activities, not just the same thing every day. Even simple things like brushing your teeth with the other hand stimulates brain activity.
No one knows. No one knows for sure, though many are trying to find out. It is theorized that continuing to challenge your brain in new ways, including learning new things, may delay the onset of alzheimers.