Concord is relatively easy to navigate by car or public transportation. BART (Bay Area Rapid Transit) has stations in Concord that can take you to different parts of the Bay Area. There are also several car rental options available at Concord's airport. Expect to pay around $20-30 for the car rental or $5-10 for a BART ticket.
If you have more time, consider visiting the Concord Pavilion or the Six Flags Hurricane Harbor water park. If you have less time, skip the Markham Regional Arboretum and add a visit to the Gardens at Heather Farm instead. Also, be sure to bring comfortable walking shoes and sunscreen for the outdoor destinations.