Pick an Exercise
The best exercises for Greasing the Groove tend to be body weight exercises like pull ups, push ups, dips and air squats. The kettlebell swing can also be used. The key is to make it easy to do reps throughout the day.
Pull ups are great but if you don't have a pull up bar handy, how will you do it? Same with dips - you need a dip rack.
One of the many beauties of the push up is they can be done anywhere. Same with air squats. And it's also pretty easy to bring a kettlebell with you (to work or wherever you go) and have it available.
So pick something you want to get better at that can be easily executed throughout your day.