I recently came across chalk board paint for the first time. Two coats of paint and literally anything can become a chalk board! It's quite cheap as well, so that's a bonus!
We always have a graffiti board as one of our Sunday morning prayer stations, but to give us a little bit of a change I've swapped the cardboard for a chalk board. All I did was paint one of the random bits of hardboard left over from the building work and now we have a five foot long, reusable board. The children love using the coloured chalks as well. It's always good to have some variety in prayer!