Something to be aware of is that people with disabilities, in particular hidden disabilities, may not appreciate a Slack bot that brings their physical activity into a Slack channel. An opt-in mechanism would go over better than an @channel.

You work hard. Take 20 seconds to stand up and stretch. You can do anything! Do a pushup, squat, jump, or walk to the water cooler.

Whatever you do, invite your team. It is more fun together.