While we don't have direct accounts from Albert Einstein himself on his absolute favorite thing to do, we can infer some possibilities based on his known interests and personality.

Here are a few contenders:

* Thinking deeply about the universe: Einstein was renowned for his intellectual curiosity and dedication to understanding the fundamental workings of the cosmos. Spending time pondering complex scientific concepts and developing new theories would likely have been a deeply fulfilling activity for him.

* Playing the violin: Einstein was a passionate musician and often found solace and inspiration in playing the violin. He believed that music and science were interconnected and both held a profound beauty.

* Sailing: Einstein enjoyed spending time on the water and found sailing to be a relaxing and enjoyable activity. He often used sailing trips as an opportunity to unwind and clear his mind.

* Spending time with his family and friends: Einstein was known to be a warm and loving person who valued close relationships. He enjoyed spending time with his family and friends, engaging in conversations and sharing meals.

Ultimately, it's impossible to say definitively what Einstein's absolute favorite thing to do was. However, his deep interest in science, music, nature, and human connection likely provided him with numerous sources of joy and fulfillment.