main trainstation is open sundays from 3-8pm. not sure if that fits you.
generally I would expect an a4 as a plat with ilmr booking. maybe e200/e220

airport has much bigger choice. they have at lesat 50 cars all the time in stock.

but trainstation also has convertibles, but you have to ask them.

for cxmr booking at trainstation I got anything between z4, e220 and audi a4, volvo s80, renault convertible, slk with plat. really depends on the booking situation and the weather.