Start a project that you can work on when you are together. Build a model or a piece of furniture or a go-kart etc. You can then set a plan in place that ensures regular activity on the project when you are with your children. This can be an additional conversation point on what is going on in their life (particularly as teenagers), and helps in trying to get them away from the inevitable conflicts of electronic games etc. encroaching on time.
There may be tasks that your child can do even while you are absent, if you set them up beforehand. It gives you a topic of shared interest to discuss when you are away from your child and it gives your child something to look forward to when you are back together.
The project also helps to set goals and create time commitments when you are with your children, that both you and your child must look to achieve.