Idle cash in Trading account

I have idle cash in my trading account for 2-3 days.
Is there any instrument where this money can be used? Like any liquid etf or anything else?

You literally stated the instrument also. Put in LIQUIDBEES. Other ETFs have DP charges on selling

What about STT? Is it the same as stocks at 0.1%?

When do we start receiving interest units? From date of purchase or from the date of credit in demat account?

Actually I was incorrect. DP charges are only waived by CDSL, not Zerodha.

Other transaction charges are mentioned there.

LIQUIDCASE is a Zerodha liquid ETF. Instead of paying interest, it grows in NAV. Making it very easy to track and avoiding headache of interest and fractional units.