This is personal. But you should get to know your clients over time. If you have clients who are more "subscribers" paying $15 per month for example you wont have the opportunity to know them. You could give them a free month, or give them a discount, or a promotion just to thank them.
If you have big dollar clients that took you a long time to win, then you want to give them something more personal, and substantial.
In my car business days we would give out Plasma Tvs, vacations, golf clubs, and even once an antique bible to partners we had. (These partners were the clients for our loans). Then for our customers we gave away Ipod nanos, but only for new customers in the month of December. Both campaigns were highly successful.
You might send out an email featuring a cute & colorful "holiday graphic" (ie- digital postcard). It's a nice way to send them a holiday wish and keep your business fresh in their minds too.
For the occasional newsletter like this, Campaign Monitor is quick & easy.