Here are some ideas:
National Garden Month: Document your garden, seed purchases, plans, etc. (side note: The Latin word aperio, meaning "to open or bud," gives us the name April)
National Humor Month: Take a photo of something that makes you laugh.
Yellow: Look for all things yellow in your environment this month. Or use yellow papers and embellishments to brighten things up.
Grilled Cheese Month: Make grilled cheese for lunch or dinner and take a picture.
Home Improvement Month: Take a pic or document a home improvement project.
Spring cleaning - make a list of your spring cleaning tasks, a before and after shot of de-cluttering, etc.
New life - take pictures of buds sprouting, baby animals, spring flowers, etc.
If you have any ideas for April, please post here.