Solo travel can be the ultimate in self-indulgence; you can rest when you want and pour it on when you're feeling ambitious. Another benefit is that your mistakes are your own, and your triumphs all the more exciting. There's no worrying that your insistence on trekking all the way across town to a museum that was closed ruined your partner's day; it's your own day to salvage or chalk up to a learning experience.
Also, you can do exactly what you want to do -- all the time. Always wanted to try surfing? Sign up for a class and go for it; there's no one sitting on the beach bored while you have the time of your life. Have no desire to see Niagara Falls? Just drive right by