Mumbai might be my favourite eating city in India, and almost none of the good stuff is fancy. This is what I'd put in your hands, roughly in the order I'd feed it to you across a day.
Start on the street
Vada pav is the whole city in one bite — a spiced potato fritter in a bun, for the price of basically nothing. Then find a pav bhaji stall and don't overthink the amount of butter involved; that is the point. For the evening, Bademiya behind the Taj does seekh kebab rolls that have kept me coming back for years.
A couple of sit-down classics
If you want one proper old-Mumbai meal, Britannia & Co. for the berry pulao — it's Parsi, a little theatrical, and unlike anything else. Leopold or Cafe Mondegar in Colaba are touristy but genuinely fun for a cold drink and people-watching.
The honest notes
Spice here is medium by Indian standards — very manageable. Street food is fine if the stall is busy and the oil's actually moving; skip the sad, empty one. Carry small cash. And if your stomach isn't used to India yet, ease in — don't make vada pav, kebabs, and a lassi your first three hours on the ground.
