There are plenty of places to park in Downtown Orlando near Lake Eola, including metered parking, parking garages, private paid parking, and side streets.
Parking meters are enforced on Saturdays.
The event takes place on the east side of Lake Eola Park (opposite from the Amphitheatre) bordered by E. Central Boulevard, N. Eola Drive, and E. Robinson Street. Here is a map of downtown parking locations. Here is parking information on Downtown Orlando.
Biking to Earth Day is strongly encouraged! There are plenty of places at the park to lock up your bike.