We take our annual beach trip early in June each year. Last year, we had the idea of taking some Father's Day themed photos while at the beach. I posted the photos last year, but failed to provide the details on how we took them.
Here are a few tips to help get the best looking message in the sand photos possible. First, unless your kid is older and really careful, let them play on the beach while YOU write the message. Getting the letters to show up was harder than I thought it would be. I got decent results by adding some water to the sand and using a stick. Some of the letters were still a little hard to see. Write a little and then check to be sure that the shadows are working in your favor to make the letters easier to see. If they are still a little light you can bump up the contrast with photoshop or something similar after the pictures are taken.
If you are writing something long, like "Happy Father's Day", perspective (the concept that things get smaller as they move further away) will become an issue. Splitting the message into several lines will help, but you may also need to purposefully make the words bigger as they move away from the camera do that they won't appear terribly small in the photo. Once you have your lettering figured out, you will probably have a ton of footprints around it. Gently brush the sand until they aren't noticeable and back away, hiding your footsteps as you go. Hand your kid a beach shovel and set them in a good location for the photo. Take tons of pictures, it can be hard to see if you got a good picture in the bright beach sun but you can delete the extras later. Other than that, print your pictures and either frame them or attach them to a card. Simple and sweet!