I will be on a cruise in April 2019 stopping at Mumbat for 2 days. What is the best plans I can make to make the most of my 2 days stay in Mumbai?
Check out your Roll Call on the CruiseCritic forum for your cruise line and ship. You'll likely find that your fellow cruisers se planning private tours that you might like to join. Also check out the Ports of Call forum. Aside from that, without knowing your particular interests, it's difficult to know what to specifically recommend.
In Mumbai, you can visit the famous food street also known as Bombay Chowpatty or you can visit a few historical sites like the India Gate or visit the pubs Mumbai does have a good nightlife to enjoy. Just don't miss the local food out there.
Mumbai is not particularly historical, and perhaps the least Indian place in India, and there's absolutely no "must see" attraction. But there are a few minor things to do. In addition to the Gate of India, there's the cave at Elephanta (boat ride) which pales in comparison to the cave temples at Ajanta and Ellora but you would need at least 3 days for them, although here is a strenuous 2 day trip https://www.trawell.in/itineraries/mumbai/2-day-trip/ellora-aurangabad-ajanta
There is a great tour company in Mumbai, Mumbai Magic, that we used when we docked there in 2015. I highly recommend them. They are not expensive.
Doing Mumbai on your own, I would not recommend that. Traffic can be very heavy and some parts of the city are not very safe to enter.
Since you are there only for two days, I suggest you cover places like-
1) Gateway of India
2) Elephanta caves
3) Siddhivinayak Temple
4) Juhu Chowpaty
5) Sanjay Gandhi National Park
Another thing you can do is book a bus for "Mumbai Darshan". They are the MTDC buses managed by the government that provide a daily tour of the best tourist attractions in the city.