All these items, I found at our local dollar tree, except for the old sheet, and I bought that vintage children's flat sheet at a local thrift store for $1, washed it and viola! I made this Fort Kit for $11 total.
First buy a "Knapsack"
Print out what you want your "knapsack" to say using Word or other program on your computer. I printed it on cardstock so that when I used a Sharpie to trace it, it wouldn't bleed through but you can also put a piece of cardboard or plastic in between as well.
Now fill with different items you find at Dollar Tree that fit with your theme.
Flashlight, glow sticks, candy, popcorn, hot chocolate, clamps or clothespins, rope, children's books, coloring book, markers or crayons, games, lip balm, etc.