Can the cert be used for convertibles?
The certs from ONE-TWO FREE can be used for any car up to a Full Size, so basically anything in the Emerald/Executive Aisle. Depending on the location and season, there may be convertibles available there.
If there aren't any in the Aisle, you can ask a manager how much an upgrade would be (assuming they have available not in the Aisle).
*As a note, these certs are different then regular National Free Days which come from Rental Credits, those can be used for any car size including Luxury and Convertible.