Can also try sajjan garh fort .. the view from there is amazing. I would have also suggested Bahubali hills (it's very beautiful specially now it would be very green out there) but since u just have one day it wont be possible.
Hi,
Here are some places you can visit in a day in Udaipur: Fateh Sagar lake, City Palace, Lake Pichola, Karni Mata Temple, Sahelion ki Bari, Sajjan garh, Ambrai Ghat.
Here is a 24 Hour Udaipur Travel guide, that can help you plan your day trip better: https://www.tripoto.com/rajasthan/trips/things-to-do-in-udaipur
Hope it helps!