When spending time at the beach it's always nice to have a chair, but if you walk or drive to the beach carrying a big folding chair can be a hassle. Design firm Tovdesign came up with an open source design for a simple beach chair using two towels and broomstick-sized dowels.
The 'Sand Chair' is made by cutting and sewing four pockets to hold dowels onto a beach towel, these pockets can be made from any sturdy fabric although most users will probably use a second identical beach towel. When you'd like to have a seat you'll just insert the dowels into the pockets, cross the front dowels and push them into the sand, then push the back dowels into the sand. See the source link below for additional photos and videos of the pockets and chair setup.
This chair won't be the most supportive in the world -- your butt will probably rest on the sand itself, but it will prop your back up and raise your legs a little. But for the low cost and easy carry option it's much better than no chair at all and you can still use it for normal towel functions. Also since the sand chair takes up so little space it might be worth carrying if a friend comes along or you have a chair malfunction.