Let's say you have to pick up a bunch of groceries, but don't have your phone or even a pen and paper handy. Think about the house you grew up in, or any home/building that you know very well, and mentally walk a path in and around the place. Picture each item you need to buy at key spots along the path. For example, I use my parents' house:
Mailbox: visualize a box of Cheerios
Front door: bananas and milk (you can put more than one item, if they work together for you)
Piano stool in living room: roasting chicken
Living room couch: dozen eggs etc.
If you can think of the items "doing" something, like a milkman slipping on a banana peel, it makes a stronger memory.
When you get to the store, follow the path again in your mind, and you'll see each item right where you put it. The first time I tested this out, I was able to remember around 20 items along my path.
Even weeks later, I was surprised that the items were still there. You should go through and mentally "clean" the path before re-using it, because the path can get cluttered.
Even if you never use this for a practical purpose, it can be a fun parlor trick.