I keep a hose more-or-less permanently attached to the spigot on my filter return line, as I drain and refill periodically to lower CH. But that's a different topic.
The weather here has been bizarre; more like England than Southern California. So the pool just won't warm up, and I'm too cheap to run the heater. So I'm out puttering, and get the brainstorm last week and laid said hose on the patio, where it can get hot. The water's still coming and going to the same place, but this way it's getting warmed a little.
Today, the sun finally came out of hiding, so I cracked the valve a little and have it warming again. I also have a black, 100' hose attached to the hose bib on the house. I need to add another inch or so to the pool, so I dragged that whole hose out on the patio and have the water just trickling into the pool.
Cost? Running the pump outside the normally programmed time. That's it.