We did an expedition out there a few years back, I think we lowered everyone's property values by having the shittiest boats on the lake.
There's a semi-active scout camp built on some of the park foundations, and a few ruins around the island. Nothing really large left of the park, although there was a little castle/fort thing on the beach. A few people found old coins and trash that might be from the park days.
Big Island is major bro-boater territory during the summer. Apparently when you have a boat on Minnetonka you are required to drive to Big Island, park in shallow water, and wade around your boat buffing and waxing it with your shirt off while your trophy wife suns herself. Typically everyone on the lake does this at once, I think it's a law there or something