You have to go under the category "Jeep/Cabrio"
Their fleet does have several convertibles listed. If you click "PKW-Flotte" (passenger car fleet) they have:
VW Beetle Cabrio
Renault Megane Cabrio
BMW Z4, Audi TT Roadster
BMW 3er Cabrio, Audi TT Roadster
Mercedes Benz CLK Cabrio
Here's what I got for June 11-June13
Tarif:
Jeeps / Cabrios - Tagestarif
Abholung (pick up): Düsseldorf Airport - 11.6.2006 / 12:00
Rückgabe (return): München Airport - 13.6.2006 / 12:00
Wählen Sie aus der Angebotsliste (choose from the following list)
Cabrios
€ 198,00* VW Beetle Cabrio
€ 254,00* Renault Megane Cabrio
€ 284,00* 0DR/0PSGR
€ 294,00* BMW 318/320 Cabrio
€ 318,00* Mercedes Benz SLK
€ 350,00* Mercedes Benz CLK Cabrio