Grundy is a well kept small park, there are some very short portages between the lakes which are noted on the map in the link below, and one longer push through some reed beds to get to another lake. There are two islands you can camp on but they are first come first serve and are canoe-to only, the other spots are car camping. You can catch Walleye (not Pickerel) and Pike, you may see Moose, Deer, and Fireflies (remember those?). All lakes are no-motors, not even electric. I suggest you book a site before going as it is a popular park, I recommend a site adjacent to one of the lakes. you may get some bad weather as it is close to Georgian Bay so be prepared for anything and be thankful if you don't get a storm.
Here is the website that will let you book:
https://reservations.ontarioparks.com/GrundyLake?Map