See Sandy? It wasn't difficult to find out a castle with a cache near us. We just drove 20 miles to get it. It was great with the kids.
The coin is now at Sesimbra's Castle:
" When D. Afonso Henriques conquered the land from the Moors in 1165, Sesimbra Castle, darkened by the passing of ages, had long stood high on the hilltop. If had been a military stronghold dating back to Saracen times. Sesimbra had never been famous for a war-like population and the town, which is surrounded by mountains on all sides but the south, was situated far away from the bloody battlegrounds where the Portuguese and Saracens fought for what they believed was theirs."
Read more here: http://www.mun-sesimbra.pt/turismo/Visitas/castel.html
Hope this is the first of many, many castles!