Iberia have teamed up with Hertz to offer triple miles if you book a rental before 28th May 2014.
The rental period can be anywhere across Europe, needs to be 3 days or longer (hint, if you book a rental for 49 hours, then that counts as 3 days too), and needs to be completed by 30th June 2014.
The normal offering for Iberia Plus member is 300 Avios per day, so with this offer it's 900 Avios per day.
To receive your bonus, you need to use the following codes when booking (you can select your country and language at the top right):
Discount Code (CDP) – 763203
Promotional Coupon (PC) – 198365
(Hat tip: Flying Piggie)