The Riverside Convention Center offers surface parking immediately adjacent to the center.
Downtown Riverside City Parking Map

Riverside Convention Center
3637 Fifth Street, Riverside, CA 92501
Convention Center Parking Entrance: LOT 33
3698 3rd Street, Riverside, CA 92501

Convention Center Parking Entrance is at Third & Main Street
Daily Cost $15

If you are staying at the Marriott: 
Parking Fee is per night per vehicle / Garage height clearance 7 feet
Fee: $ 25 USD per stay

Downtown Riverside
Parking and app info (map, garages, lots, metered, nightly rate) can be found on their website.