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 not enforced on Sundays.
The event takes place on the northeast side of Lake Eola Park (opposite from the Amphitheatre) bordered by N. Eola Drive and E. Robinson Street. Please keep that in mind when parking. 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.