Whether you are designing your summer house for the entire family, or simply a place to invite, or rent out to, guests, those aspects will impact the creation of the spaces according to their intention. A summer vacation beach house will also be different from a winter cabin in the mountains. The first usually being more outdoor-oriented, with light tones, and fresher fabrics for upholstery and drapery, and easier to clean (especially if you have a pool or a beach nearby). Then the latter could incorporate a fireplace, warmer tones, and furry soft goods to make the place feel cozier and more welcoming.
If you intend it to be for family use, where different personalities and age groups will congregate, then it is important to make it simple and add some fun elements that everyone can enjoy. These are elements that even if you wish to rent it out, will still hold up nicely.