I'm a bit of a completionist when I have the time available to me. When I play open-world games, I tend to go region by region, clearing one before moving onto the next. When a game offers a fork in the road, I tend to choose the one that doesn't look like it'll move the story forward. I want that sweet, sweet hidden stuff first.
God of War is a game that taunts people like me. You see treasure chests and alternate paths, and you assume you can access them with your current tools. You cannot. God of War is a game that wants you to return to areas much later to get all those goodies.
As such, I feel it's my duty to warn you that God of War isn't going to lock you out of its side content if you finish the main plot. There are a host of treasure chests to find and special armor you can unlock by finishing specific challenges. Many of the chests you saw during your adventure will be available to you once you have all of the additional abilities, it's just a matter of remembering where the chests are.
There's more to the endgame though. God of War's adventure goes directly through Tyr's Temple, which is a gateway between the Nine Realms. A number of the realms are unlocked in the course of your adventure, but you'll have to find Cipher Pieces in order to unlock another two of them.
Muspelheim, the realm of fire, offers a series of Arena trials to complete. Niflheim, the realm of fog, is more of a dungeon crawler, forcing players to kill the realm's inhabitants for the resources to craft the best armor. Both of these realms can be accessed and tackled before you finish the story, but they definitely feel like endgame challenges.
There's still more out there. Kratos and Atreus can find the hidden prisons of the Valkyries, which are extra-difficult bosses you can tackle. Two are found in the realms listed above, while the others are in Hidden Chambers of Odin. Finish them all to take on their Queen, one of the hardest encounters in God of War. There's also a few dragons out there. While you'll tackle some during the story campaign, there are a few you can free in certain locations around the world.
Basically, just because you're done with God of War doesn't mean you're done with God of War. There's still a ton more to do within the world of Midgard and even a secret ending that some folks might miss if they don't explore everything.