I think your party is such a lovely idea. As an exhausted mom of 15 month old busy boy and pregnant with our 2nd, I think your idea of snacks, drinks, and music sounds like a dream. I don't think you really need to add anything else. I imagine you all will enjoy the adult time and have some nice conversations. It might just be a great mom evening without adding anything else to it. If you wanted to, you could always just do a theme like a wine tasting and cheese or a mexican theme with chips, salsa, some sort of easy mexican food and margaritas.
Have a great gathering!!