My granddad was a marine in WWII. I was fairly young when he died but he never would say a whole lot about what he did. I’m pretty sure he was in Japan but I’m not sure. Is there a way I can find out what he did?

You can go to the VA and request records. Many counties have offices in the local courthouse

I sure ain’t gonna debate the point as the war in Africa and Europe get lots of deserved attention, the Pacific was a MutherF5#!

Salute

I wish I could get my Grandaddy's service records from WWI but they were stored at a place in St. Louis that burned some decades ago and there were no other copies. Lots of records lost forever

If I remember I went here:

https://www.usa.gov/military-records

I requested some records and they sent them to me, and then weeks later I received a package in the mail and it was my Good Conduct medal that I had never previously received. I thought it was pretty cool of them, almost 40 years later to send it to me.

It May be a long shot but the VA may or could assist you with getting a copy of his DD-214

I don't know how much help this will be but here goes: If your grandfather's records were destroyed in the fire in St. Louis, the State where your grandfather was born and/or enlisted may have records in their Archives Department.

I was able to locate some information on my grandfather's draft registration for WWI and and I found some info on my great uncle's WWI service when I found the request his sister (my grandmother) made for his grave marker in 1935. I used that info to search information about his unit, etc. I found these on the LDS website
There are associations and some facebook pages that have information out there. Sometimes you can get info from there. I found information about another uncle for WWII through the Glider Pilot Association.

Good Luck.

Depends on what details ou are wanting to learn, if you can find out what company, battalion, division. You could find out what theater he served in, possibly find an old Marine still alive from that war, but they are leaving here fast. There is a group called together we served that might help, good luck
Semper Fidelis