Non-exempt in California: Is it considered adequately paid overtime if I work a 6th day, but as compensation I am paid $100 (less than time & a half) in addition to getting a paid day off (at the regular rate) at a later date? I know straight comp time is not equal to overtime, but what about payment plus time off?

If you are a non-exempt employee and you work for a private employer, then any form of time off in lieu of overtime is illegal.

Agreed. And according to a 1992 Wage and Hour Administration letter ruling, the employer cannot claim an offset credit for the "compensatory time" off given to the employee in lieu of payment for overtime, and must instead pay cash compensation for such overtime hours notwithstanding any paid "compensatory time" leave taken by the employee. See 1992 DOLWH LEXIS 28.

You can file a claim with the DOL to recover your overtime pay at time and a half, or you can contact an attorney to pursue recovery of your overtime pay at time and a half plus an additional equivalent amount as liquidated damages under Section 216(b) of the FLSA.