Several issues ago, in an article titled “Posh Backyards,” we covered the topic of unique upgrades that you can make to your home’s outdoor area to create your own version of a theme park, complete with lap pools, putting greens, observatories, outdoor kitchens, and saunas. The response was good, so now we’re shifting our gaze inward, to make the home itself your very own retreat—essentially a suburban Shangri La. When visiting friends or staying at a resort, it’s common to see things you wish you had at home, such as a huge walk-in shower in your hotel room or a neighbor’s lavishly decorated entertainment room, equipped with a pool table and arcade games. The common reaction is to say, “We’ll add that to the new house,” but that could be months or years from now. We have a better idea: Do at least some of the upgrades with your current …