I love this catchy song - it definitely helps the kids remember this concept.
GuruParents have a great range of videos - we would watch these weekly when trying to brainstorm words for our sound of the week board.
When Two Vowels Go Walking
Between the Lions have lots of great ideas in their videos - I really like this one for illustrating what happens when there are two vowels together.
Again, another range of videos which have lots of ideas for ways to show your kids different concepts.