You can do this by running an Arrivals Report.
Select your applicable dates of stay (e.g. are you sending a card to guests who stayed over the summer, or the whole of the year?)
Tick the ‘Address’ box in ‘Extra Columns’.
Export to a .CSV file. You can now filter the data so that you only send cards to anyone who has stayed over a particular timeframe e.g. a week (period), or spent over a certain value (balance) or in a certain accommodation (unit name). Eliminate any columns you don’t need and voilà, there is your customer name and address list.
If you don’t want to hand-write addresses on envelopes, you can follow these instructions to change your output into printable labels: https://support.microsoft.com/en-gb/kb/318117.
Alternatively, you can just run a campaign out of your e-marketing module.