If both of you are big fans of the outdoors, there are a lot of places in Mumbai and even close by, for a one-day outing. You can drive down to Kasara, and camp out by the Bhandardara Dam, and spend a day boating, hiking, or simply unwinding. At night, there's always stargazing and campfires.
P.S: It’s pocket-friendly, it’s fun and you’re sure to make happy memories.