Depending on your budget, here is what I suggest:
Start the day by going out for breakfast. Next, stop by a nursery and let him choose some nice flowers or herbs. Also, you could pick out a few pots for them to be planted in and soil. Then, you can plant them together and will be able to watch them grow over summer..
I really liked the other mom's ideas for a picnic and pool.
Another idea would be for you to take a camping trip over the weekend.
You might also see if another mother and child pair wanted to team up for an afternoon hike or dinner.
This might be stretching it, but you could head down to the Monterey Aquarium.
Happy Mother's Day!