Best one-day outings from Mumbai?

Avoid Lonavala. I was there couple of days ago, more polluted and warmer than Mumbai.

Same with Silvassa.

Nashik seems to be doing better.

I think a trip to alibaug would be good ..it's short ...plus the beaches are fairly decent

Igatpuri, Nashik for its Misal Paav, Murud-Janjira sea fort, Nagaon beach for water sports, many forts around Pune, Matheran hill station for the toy train ride (check schedule first), Bhandardara. If you can spare an extra day, I believe Mahabaleshwar would be better than all the previous options as you have Paavna lake camping, strawberry farms, Kaas plateau which is full of orchids during September - October and many more things to do.

If you’re looking for something within Mumbai then there’s GoKarting in Wadala

Vaitarna lake