If it is the six flags in Los Angeles, then they have sushi there... There is also a grilled chicken sandwich... It is probably plumped up with salt water, but I am sure it is better than a lot of the other options. Good Luck - Eating healthy in a place like that can be really tough.
I've never been to six flags so i'm not sure but have you tried looking up the food vendors that are there? Then try to google their nutritional information? Also most of their food is probably frozen most likely so you could try foods like pogo corn dogs, popcorn i'd go with actII or orville, these are my guesses if i had to work with nothing.
Here's their food list so from here the big names will have nutritional information on their own website. This is from new jersey i'm not too sure where your from but there should be a similar site for your city.
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