Tour guide or nah (for Sintra) and/or Cascais

We enjoyed a day trip to Sintra and Cascais with Lisbon Riders. We departed from Lisbon.
They take up to eight passengers in an air conditioned van. Our guide was Manuel. He was knowledgeable and engaging. We highly recommend this guided journey. www.lisbonriders.com

Sintra is very easy to visit yourself as a day trip from Lisbon. There are regular direct trains. The key is planning your day carefully. There are a lot of tourist sites to see, and a few of them are up on the hill above the town. There are public buses to get up there from the train station. There are also "tuk tuk" 3-wheeled "cart" taxis to take you up there (and elsewhere in Portugal) if you prefer.

I'd get your tickets to Pena Palace, assuming you will go there, ahead of time - don't just show up at the palace park entrance or you can expect to wait a long time in the lines. Alternately, you could visit the Moorish castle ruins first (if you want to go there) and buy your Pena ticket there to skip the line later.

Don't miss the Quinta da Regaleira - it's an unusual park/garden complex with a mansion (forgettable) with some neat features. It's down at the town level, an easy walk over from the center of town.

Cascais is a lovely kind of upscale beach town. It's probably easiest to take a separate train trip there from Lisbon - your day in Sintra will probably be packed with stuff to see/do there.

https://wikitravel.org/en/Sintra

thank you everyone - all your suggestions are quite helpful:)