To plan a core strengthening workout for couples who want to stay indoors, start by setting a specific time and day for the workout. Choose exercises that target the core muscles such as planks, Russian twists, and leg raises. Make sure to warm up before starting the workout to prevent injury. Create a circuit of exercises that can be done together, alternating between each exercise for a set amount of time or repetitions. Consider adding some partner exercises like medicine ball passes or partner planks to make the workout more engaging. Finally, cool down with some stretches and congratulate each other on completing the workout. With a well-thought-out plan and commitment from both partners, staying indoors can still be a fun and effective way to strengthen your core.