I was reading a book on the beach today, when I happened to look up and see this lady nearby pull a giant plastic kitchen spoon out of her bag. Instantly I had a feeling I knew what she was going to do with it, and a few seconds later my hunch was confirmed. She squirted a large glob of sunscreen on it, then proceeded to rub it all over her back.
To me it didn't look as if the spoon was a particularly effective tool for this purpose, but I'm also hard pressed off the top of my head to come up with anything better. My first thought was to get something like those brushes that have dish soap in the handle and fill it with sunscreen instead, but obviously you'd want a device that wouldn't absorb any of the lotion itself.
And from watching this women it was clear today wasn't the first time she'd done this, so I'm betting over the years she'd probably explored a bunch of other options before settling on the spoon. Apparently one thing that didn't occur to her though was asking her husband sitting next to her for help.