This will be my 3rd trip to Portugal. I have been to Cascais, Lisbon, Porto, a fast day trip around the Duoro and the Algarve. This time I would like to explore Sintra again (it rained the day I was there. And then have a beach holiday near some walking trails and a historical town. Am I asking too much? Would love some suggestions.

I don't know about walking trails, but I'd encourage you to think about at least two full days in Sintra -- there's a LOT to see there!

KJA am I remembering correctly�you have a trip report?

Not on Portugal. But there are a LOT of TRs for that delightful country! And I'm happy to offer comments on planning threads when I can.

Since you have asked for suggestions, my strongest one would be that you state when you will be going to Portugal. I realize that your reference to beaches suggests late spring, or summer, but in principle, readers who see your post two months from now won't know whether it's still worth their time to compose responses.
I'll add that as to whether you are asking too much, I would say definitely not -- you have proposed Sintra, beaches, an historic town, and hiking trails; you can definitely take in all that, and more, in a typical two-week visit, and maybe even one week. For example, Lagos is an historic town with beaches near-by, and a relatively short trip from Lisbon. One night in Sintra, probably another in Lisbon, and four in and around Lagos, and within six nights you will have achieved three of your four goals. And within that same Delaware-sized province of the Algarve, you will probably find hiking trails in the Serra do Caldeir�o. [Note that though I have been to Portugal, I have not seen the Algarve -- I mentioned those sites not necessarily to vouch for them -- though I'm sure they're fine -- but only to illustrate that you are not asking for too much!]

Thanks for your input.
It will be during the summer.
As I mentioned in my post, I have been to Portugal twice. Apart from flying in, I think I will give Lisbon a pass this time around.

My choice would be Ericeira. Very close to Lisbon (I agree, Lisbon is been there , done that with too many crowds) Excellent hiking abounds on the cliffs overlooking numerous beaches/coves directly from the old town. An old fishing village, not a lot of tourists. “Surfing capital of Europe”. Tons of cafes, restaurants, and shops and the whole town is cobbled stone streets, with the houses dressed in blue and white. A 10+

We loved Evora and when our plans changed, we went to Marvao-- beautiful views. Castelo de Vide - lovely town. Thomar Boutique Hotel. We did not have good weather, but these villages and mountains would be wonderful in good weather! Trip report-https://www.fodors.com/community/europe/wonderful-portugal-1713270/ Look at days 5 and 6 in particular.

Thanks so much for these suggestions. I am looking into them.

Check out Along the Costa Vicentina - Rota Vicentina and especially the Fisherman's Trail.
The Costa Vicentina (westcoast of Portugal, south of Lisbon) is wild and unspoilt. The beaches of golden sand are fabulous, surrounded by steep cliffs.
There are a lot of small, typical villages such as Vila Nova de Milfontes (which is where we stayed), Zambujeira do Mar, Porto Covo, ... and far less touristy than the Agarve.