I'd try to find her something she really likes-a dvd, cd, movie tickets, her fave lotion, etc. As a mom, & especially when you're expecting another one it's hard to 'justify' buying yourself anything just for you. But if you do it I'm sure she'll love it.
If you're able/interested, something I've added to my sister's gifts before are 'coupons' for free babysitting. Maybe to be redeemed the day/night she enjoys your gift.