Unfortunately, there are only 24 hours in a day and, if you're lucky, you will spend a good chunk of those hours sleeping. So you won't be able to do everything. But it'll still be wonderful.
Let's break it down into sections.
Morning
You're up. The kids are up. You need coffee and you've got lots of options.
Linnaea's Cafe 1110 Garden St., SLO
Great coffee, tasty treats, and a beautiful garden in the back.
Scout Coffee 1130 Garden St., SLO
Charming coffee shop straight out of Pinterest.
Kreuzberg Coffee 685 Higuera St., SLO
Stylish coffee place with lots of tasty food!
SLO Donut Company - SloDoCo 793F Foothill Blvd, SLO
Bacon covered maple bars!
There's also Blackhorse Coffee, Big Sky Cafe, Bliss Cafe, Bello Mundo, and Sally Loo's. They're all pretty great.
Maybe you want to get some delicious fresh fruit? Go to the Farmers Market on Saturday morning from 8 to 10:30am. It's full of strawberries and apples and everything else you could want from a farmers market without the circus of SLO's Thursday Night Farmers Market. It's in front of World Market on Madonna. That's where we are every Saturday morning.
After you get your coffee and breakfast, you might want to roam around downtown SLO. Take those kids to Eco Bambino on Monterey St. right next to the Mission Plaza. You'll love the toys and try to justify the prices. Then wander around Mission Plaza Saturday morning for Via dei Colori as artists color the ground with chalk art. Take your kids down to the creek and let them dip their toes in the water. Then pop back up on Higuera St. for Tom's Toys or go across the street for Rocket Fizz and load your kids up on sugar. Y'know. Cuz kids!
Don't take too long downtown because you'll want to go up Monterey St. to Ruby Rose and their annual flea market.
Afternoon
For lunch, get some chowder at Splash Cafe. Just do it. You know you want to. Just don't go to the one in Pismo. I mean, it's great and you'll get that flash of nostalgia, but it'll be so busy! Do yourself a favor and go to the Splash in Downtown SLO. Afterward, maybe hit up one of the many frozen yogurt places, Doc Burnstein's Ice Cream, or check out Batch for ice cream sandwiches.
Then, get over to Avila Valley Barn and check out some farm animals, eat some roasted corn on a hay ride, and maybe do some old fashioned apple pickin'! By now, you'll certainly want to go cool off so cross your fingers for parking in Avila Beach and dunk your feet in the ocean.
Other Great Stuff
There are lots of great places to hike, but it's gonna be warm so I'll keep the list short and relevant. For a quick view of downtown SLO without the struggle, go to Terrace Hill (5 minutes tops, park on Bishop). If you find yourself in the Morro Bay area, you have to check out Black Hill. If you're in Avila, drive up Cave Landing Road and check out the cave! If you're feeling more adventurous, I'd suggest the Elfin Forest (Los Osos), Los Osos Oaks State Reserve (Los Osos), Bluff Trail in Montana de Oro (Los Osos), Johnson Ranch (South of SLO), or the Hike to the "M" if you're real crazy (45 min. roundtrip).
For food, a few personal favorites are Brickhouse BBQ (Morro Bay), Sylvester's Burgers (Atascadero, Los Osos, or Oceano), Pluto's (SLO), Firestone BBQ (SLO), Woodstock's Pizza (SLO), High Street Deli (SLO), and Chop Street (Pismo).
Mmm. I'm hungry.
Ah! I almost forgot. Red Dirt Coffee House down in the Arroyo Grande/Grover Beach area has really good Kona coffee and on Saturdays does the best Hawaiian plate lunch I've had since Maui.
Also, best place to get a six pack of beer is definitely California Fresh in Pismo. ;)