I'm a fan of romantic vacations, but I'm also totally in love with this idea. When you travel alone free of other distractions, you have time to reflect and really engage with the place you're visiting. You can meet new people, spend as much time as you want in a museum, and revel in your newfound independence. And if all goes well, at the end you'll be ready to trade in adventure for time with the partner you're missing.
While some people try to spice up routine relationships by making them open relationships, maybe all they really need is a trip without their partner.