The best caption for the photo would depend on the content of the photo itself. However, if the photo depicts a relaxing or family-oriented activity in a natural setting, "Vamos al campo los domingos." (Let's go to the countryside on Sundays.) would be a fitting choice. If the photo shows a religious gathering, then "Vamos a la iglesia los domingos." (Let's go to church on Sundays.) would be appropriate.
If the image features a pool or swimming activity, "Vamos a la piscina los domingos." (Let's go to the pool on Sundays.) would work best.
Without knowing the photo's content, it's hard to determine the single best caption.