Assume that the future can change based on your actions, so any historical information that you bring along with you from the intervening 25 years may quickly drift out of the new realities history.
Edit: also assume that you can be given a healthy 21-year-old body if you want or take your previous self’s place.
Further, identification will be provided for you if you were not born at that time.
I’d take the entirety of the USA’s military hardware. Nobody said how big the backpack was.
You have to be able to walk 100 meters while holding onto the backpack, it has to be strapped to your back, and it has to be a commonly available backpack of a variety purchaseable in a store with the next 25 days.
No part of it can touch the ground, nor can anything attached to it other than your feet and shoes touch the ground, and everything that you have must be fully enclosed by the backpack, and you must cross through the portal on your own two feet.