Cintra is a fascinating place, largely for reasons nothing to do with the Napoleonic Wars. However, it did give its name to the agreement between the British and French made after Vimeiro which was much criticised in Britain and Portugal when news got back. The subsequent enquiry examined the involvement of Wellesley, Burrard and Dalrymple. Wellesley was able to argue that he had not been consulted about the terms and was acting under orders but it could have ruined him and the history of the Napoleonic Wars would have been different. The other two never received an active service appointment abroad again and George III issued an official censure against Dalrymple.
Wellesley's political connections helped him recover and he was officially thanked in the House of Lords for his victory at Vimeiro.
The Convention of Cintra remained a shameful agreement and was immortalised by Lord Byron in Childe Harold.
The actual location of the signing of the agreement is disputed. It's likely that it was signed in several different places.
The Palácio de Seteais in Cintra (now a hotel). This was Dalrymple's headquarters where he wrote the dispatch that accompanied the agreement on 3rd September 1808.
He ratified it on 31st August, here perhaps?
Palácio Nacional de Queluz, another candidate for the signing. There are at least two large formal rooms where this could have happened.
Palácio Nacional da Pena, (Pena Palace) fantasy castle on a hilltop. Started in 1839, so not Napoleonic at all, but worth a visit.
Palácio Nacional de Sintra (Sintra National Palace). Long time royal palace.
Another place worth visiting if you are in the area is Monserrat, recently restored with may British connections including William Backford, Malcolm Muggeridge and Leslie Howard.
There are gardens in different styles and a lavish house, rebuilt in a style inspired by the Brighton Pavilion.