Just a quick heads up: DO NOT do crunches. It's not as bad as doing squats with improper form, but it's close. The minimal at best fitness benefits weighed against the potential damage makes it not worthy of your time or effort. If you guys are gonna put the work in, I wanna see you have some success. When it comes to the basic of abdominal workouts, functional sit-ups and planks/side planks are great. Those things are always great, just get more complex and are part of a longer list of exercises as you get more fit. Push-ups are terrific for your entire shoulder complex, and so are Pullups. That covers your chest, shoulders, biceps, triceps, lats, and multiple back muscles.