On a budget, Quinta da Regaleira still earns its price.
~€12
The most fun to explore of Sintra's sites — a Masonic-symbol-laden fantasy of tunnels, towers and the famous moss-walled Initiation Well you spiral down into. The catch: it too gets very busy, the well backs up with photo queues, and the grounds are a hilly scramble. Often the favourite of those who do both this and Pena.