Tween girls love this! Get out the scrubs, lotions, soaps, nail polish, and tubs of warm water. Have the adults do foot massages, nails, and hand massages. Turn on some music (either relaxing or totally fun) and start the party! The food can either be classy (delicate desserts and crackers with cheese) or very casual (hot dogs or pizza with cake and ice cream).
Hint: you should probably do the spa activities outside or on tile. If the water spills, it makes a mess! (I know from experience...)