You could drive on Worli sea face link
Check out for hanging gardens
Walk as long as you want in marine drive necklace road
Try ferry ride at Gateway of India
Sunset at Bandra and Juhu is great Nightlife is great in mumbai with flea markets to stroll by.
You could catch sunset at one of the unpopular beaches like rajodi or suruchi beach. Not many know of these! If you travel a bit offbeat, there's an evergreen waterfall at kalmandavi in jawahar, you could swim there. You could also visit juhi beach, soho house in the night. It's a stretch of land with water on both sides and place to sit in between, with the whole place lit with fairy lights. Less crowded at night. You can also visit the Asiatic Library. it's one of the largest in India and a nice place to chat at night on the stairs in the moonlight.
If you are looking for adventurous, kindly travel by local trains here. It's cheap but the experience is enriching!