Many travel books suggest staying in Lisbon and taking a day trip to Cascais. I'd suggest the opposite -- stay in Cascais and take day trips to Lisbon. While there's not much to do in this town, it is very picturesque and relaxing. We were there in October so going to the beach wasn't an option, but walking along the beach was nice and, I expect, this would be even more fantastic during the summer.
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