I've used my Silhouette Portrait and a packet of rainbow patterned card for a couple of birthday cards recently.
The first one was for my brother. I cropped The Cut Shoppe's Oh Little Star* to a rectangle and welded it to a frame before I cut it out. I then backed it with a piece of rainbow card, rounded the corners and attached it to my card front. I cut a couple more stars with my Big Shot, stamped my greeting and inked the edges before I attached them with foam pads.
(*Oh Little Star was a free Christmas file; you can find it here.)
My second card was for a work colleague's 60th birthday. This time I made my own cut file by welding the number 60 together eight times. I cut it from the rainbow card and stuck it to a white background. I based the rest of the card on Stick It Down's May card sketch, but I rotated it and then played with the proportions and positioning of the sentiment panel so that it would cover only one of the 60s.