If you have good sealing lids for them, you could use them for cacheing material in the ground in remote locations.
If you ever had plans for a greenhouse or a solar wall for your house (like a lean-to greenhouse), these would make excellent heat sinks. For a greenhouse, line the barrels up in a row, fill them with water and cap them. Put a board on the top for your plants. Water has more thermal mass than rock. It will store the excess heat during the day and release it when it cools down at night. You may not even need to open the vents on the greenhouse during the day, and your plants will appreciate a more even temperature.
This is what I plan to do with my greenhouse (when I get it set up) to mitigate against wild temperature changes between day and night. It should extend the growing season way past first frost.