Antonyms for "hitting the sack" would be anything that is the opposite of going to bed or sleeping. These could include staying up late, partying all night, pulling an all-nighter, or simply staying awake. Other antonyms might be busy work or chores, such as cleaning the house, doing laundry, or completing unfinished tasks from the day. Instead of winding down, one could engage in stimulating activities like reading, watching movies or TV, or playing video games. Overall, antonyms for "hitting the sack" would be things that keep one awake, alert, and active, rather than inviting relaxation and sleep.