Exploring a new bit of coast (close-ish to home) we came across an WWII coastal defense site. Much more extensive than the usual North Sea coast Pillboxes, front & back entrance with vertical exit shaft. The kids were giddy with excitement climbing up the iron rungs built into the shaft wall.  

It'll fuel their imagination for a few weeks.  I could hear their games starting off with..."Alice, you be the enemy and i'll escape up the shaft..."