Before this activity is tried, you will need a collection of postcards which can show children how postcards are normally set out. Once they are familiar with the layouts, they can make some of their own (which they could even be sent to friends and family).
The postcards that the children make can, of course, depict any scene. However, the children might find the activity more enjoyable if the picture is relevant to them. You could use pictures from your school, your local area or from a place you are studying.
When you have your pictures, the children (individually or in small groups) can choose the photo which they like best and open it in an image editing tool. Once this is done, they can paste a message on top of the photo, such as “Wish you were here…” or the name of the place depicted in the photo.
If they have a number of photos which they like, they could shrink them and paste them all onto one postcard as a collage so that it shows a number of different scenes from the same area.
When the children are happy with their postcards, they can be printed and a message / address written on the back