Soap cleans -- it can't moisturize. But a soap can be a strong cleanser or one that's milder, and it can be paired with a matching lotion or splash to add conditioning and moisturization.
IMO, any well made, mild recipe can be used to create a spa type soap by using the right type of fragrance and designing the appearance to suit the theme. For example, a recipe of 80% lard, 15% coconut oil, and 5% castor makes a beautiful, hard, translucent white bar. Very elegant and clean looking all on its own with a nice bubbly lather and mild cleansing suitable for most skin types. Is it fancy with all kinds of expensive butters and unusual oils? No. But a soap doesn't have to have exotic ingredients to be wonderful on the skin.
Add decorative elements and fragrance to the soap to create a look that means "spa" to you -- aqua blue-green swirls and an ocean/water scent for a watery theme, bits of vanilla bean or oatmeal with a cedar/fir/spice scent for a woodsy warm theme, etc. Finish the soap with careful beveling of the edges and perhaps a classy stamp and put it into an elegant package to finish the look.