500 points in Fireworks does not earn you 500 credits, unfortunately. (Sure wish it did!) Unless you're very good at Fireworks, you might want to try Pirates or Haunted Mansion, where you at least are likely to get some credits for playing, even if you don't win.
But if the games are stressing you out as a way to earn 500 credits, take it slow and click on all of the npcs (non-playable characters) in Shrunken Ned's, Pirate Treehouse, Matterhorn, Dungeon, Emporium, plus the three ghosts inside Haunted Mansion. You can do this every 12 hours, so that could give you 320 credits in 24 hours.
If you haven't created a music mix yet in the Street Party Music game, do so. 100 credits for creating your first mix, another 100 for playing it the first time. You can go back every 15 minutes to play it again, for 25 credits each time.
Then there's that old standby; crashing your boat in Jungle Cruise to end the game as quickly as possible and earn 20 credits. (You can keep doing this as long as your sanity holds out!)
Credits don't have to be earned all at once. You can even sell items in your inventory to go towards the required amount. No time limit on that part of the quest, I think?