Seek more work. Find tasks you can help on, earn brownie points, don't offer to do anything extra that takes more than 30 minutes to get done. Don't overdo it, and make sure to also use the downtime to grab a federally required break, stretch, drink water, meditate, do some calisthenics.

The first part boosts how you're perceived by others: your bosses will take note of your enthusiasm, your coworkers will appreciate you more; this is why it's important to not overdo it—you don't want your extra effort to be the new baseline expectation.

The second part boosts your health, mood and productivity.

If you find you have more free time than these fill, consider asking your employer to sponsor certifications/continuing education in your field to further your career, or just talking with your boss about taking on more responsibilities for a raise. But still make sure to "leave room on your plate" to do the aforementioned breaks. If the money/career growth isn't an issue, consider negotiating reduced hours so you have more free time.