Between gardens and parks and trails, there are a ton of free and low-cost outdoor activities for kids and opportunities to learn about nature.
Gardens
San Diego Botanic Garden
Formerly known as Quail Botanical Garden, these gardens located in Encinitas have a lot of hands-on activities for kids including a tree house. Although there are stairs in some places, you can also get around with a stroller using alternate paths. They get crowded on the first Tuesday of the month when it is free, but as with everywhere, the earlier you get there the better. Regular admission is not low-cost ($12 for adults, $6 for kids 3-12, + $2 for parking), but annual family memberships start at $75 ($6.25/month) and include 2 adults from the same household and their children under 18. Parking is free with membership. They have a lot of ongoing events for kids (except on free Tuesdays) in their Seeds of Wonder garden that are free with admission.
Alta Vista Gardens
A low-cost botanical garden in Vista. I just heard about this place and can't wait to check it out. They have activities for kids, too.
The Water Conservation Garden
Admission to this garden located in El Cajon is by suggested donation. It is open daily from 9:00am-4:00pm. There is a docent led tour on Saturdays at 10:30am. This garden is nearly 5 acres and promotes water conservation through a series of water-wise gardens. Among it's many gardens is a Children's Discovery trail that includes hidden animals, a sand box and a potting shed. There is also a hands-on compost exhibit.
Carlsbad Sculpture Garden
Located next to the Carlsbad Public Library off Carlsbad Village Drive, this sculpture garden has a rotating exhibit of outdoor sculptures. Open Monday - Friday 9:00am-5:00pm.
Nature Centers
Buena Vista Audubon Society
Located in Oceanside adjacent to the Buena Vista lagoon, the Nature Center is open Tuesday - Saturday 10:00am - 4:00pm and Sunday 1:00pm - 4:00pm. You can borrow a pair of binoculars to use when walking along the trail. They have a story time for kids 3-5 starting up again in September.
Jack's Pond Park and Nature Center
This nature center located in San Marcos is open Saturdays 9:00am - 12:00pm and has hands-on displays and activities for kids. It is adjacent to the play structure at Jack's Pond Park. It is a short walk down to the pond.
Batiquitos Lagoon Foundation Nature Center
The Nature Center is located adjacent to the Batiquitos Lagoon in Carlsbad and is open Monday - Friday 9:00am - 12:30pm. and weekends 9:00am - 3:00 pm. Check their calendar for events such as bird counting and trail maintenance.
San Elijo Lagoon Nature Center
The nature center not only offers displays on the history and ecology of the lagoon, but information on the green aspects of the nature center itself which was made from recycled materials and relies on solar power and natural light. Open daily from 9:00am - 5:00pm. Guided nature walks start from the nature center Saturdays at 10:00am.
Parks and Trails
Mission Trails Regional Park
This nature park in San Diego encompasses nearly 5,800 acres of both natural and developed recreational acres. There are hiking trails for all hiking levels. Their Visitor Center is open every day (excluding major holidays) from 9am-5pm. They also have recurring and non-recurring kid-friendly events.
County of San Diego Parks and Recreation
This website has links to information on parks throughout the county as well as trails for hiking, and other programs and activities offered through the county.
San Diego City Parks and Recreation
This website lists parks and open spaces, recreation centers, pools, and programs and activities offered through the city.
City of Poway Parks and Trails
This is the resource to find information on all Poway parks and hiking trails.
Blue Sky Ecological Reserve
Blue Sky is a 700-acre ecological reserve with docents on hand on weekends to lead nature walks and hands-on activities. Open daily Sunrise - Sunset
Poway-Midland Railroad
This website has information, including fares and schedules, on the railroad that runs through Old Poway Park on weekends. Train rides are $.50 for kids and up to $2.50 for adults depending on which train is running.
Escondido Parks, Lakes & Open Spaces
Find information on all of the parks, lakes, open spaces and hiking trails in Escondido.
City of San Marcos Parks and Recreation
Check out this website for a listing of the 17 neighborhood parks and 12 mini-parks located in San Marcos. You'll also find information on hiking trails as well as programs and events.
City of Encinitas Parks, Beaches and Trails
This website has links to information on all of the parks, beaches and hiking trails in Encinitas.
City of Oceanside Parks and Recreation
This website has information on various parks and trails as well as community programs.
City of Vista Parks and Facilities
This page has information on Vista parks and other facilities. See the links at the bottom of the page for a map of their parks and facilities and a listing of features and amenities.