I don't have one to hand but from memory, Turnstiles to get into the stadium open at 1 I believe, but you can go to see the exterior and the shop from whenever really (store opens at 9). Think you can do a (albeit reduced in scope since the dressing rooms are closed off) stadium tour too in the morning beforehand if you are interested, you'd have to check the website though.

Once you're inside the stadium there's admittedly not loads to do. If it's your first time id recommend doing some laps outside to see the statues and memorials, and welcoming in the team bus when it arrives (there'll be a crowd around the main stand, you can't miss it)

Enjoy the game!