Belle Isle , is as nice as it's been in 20 years now, but that's not saying much. The State of Michigan took it over about 4 or 5 years ago, prior to that , the city of Detroit operated it ( they still technically own it ). I went down there last year, went to the Aquarium and the Conservatory, both are 'interesting', but the Aquarium is VERY OLD AND OUTDATED... Virtually nothing truly good to say about Belle Isle, other then it's much nicer now that it had been in recent past...
Yes, they hold the Grand Prix there... and locals complain, because they close off nearly a 1/3rd of the island for over a month. From a racing enthusiast point of view , it isn't great either, because there's only one way on and off the island . You pretty much have to take a bus , because only true VIPs and/or race officials can drive onto the island during the racing. So, even though it's only about a hour drive down there for me, and I love car racing, I don't bother.
Now, if you want something really cool in the Detroit/Windsor area... the annual fireworks display is really cool. Yet, you have to be pretty dedicated to the experience if you want to see this on the Detroit side of the river. I hear that the Windsor side is not nearly as crazy with traffic and such. I went there a long time ago, and was really quite nice in terms of the fireworks display they put on . ( I think it's June 24th this year ).
My most favorite of all Detroit events, had been the Hydrofest ( aka Thunderfest)... but it seems this year, like the first time in some 100 years, the unlimited Hydroplane class won't be there... This saddens me a great deal.
Detroit is making a comeback of sorts. The downtown area is nicer then I'd seen it in recent memory ( 40 years or so).. but it seems some things are just not the same, probably never will be.