Parking in Balboa Park is plentiful and free. Do not try to park in front of the Hall of Champions, it's most likely going to be full. Instead, when you enter from Park Blvd via President's Way, turn left immediately into the parking lot there. The Hall of Champions is directly up the grassy hill.
There is also a convenient parking lot further up the hill to the right of President's Way. On a weekend it tends to get full, however.
If all else fails, park across Park Blvd. in the huge lot adjacent to the hospital complex.
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Note that Google put the Hall of Champion marker on the wrong building as of Dec. 30, 2009. HoC is the large, square, angled building.