My favorite gardening magazine is "Fine Gardening." It is well written and is good for beginners as well as more experienced gardeners. The pictures are always gorgeous and every issue has something either on containers or that applies to container gardening.
Also, ever year the editors of "Fine Gardening" publish a special edition on container gardening that is totally awesome and costs $7.99.
I can't imagine anyone, with even the slightest interest in gardening, wouldn't be thrilled to get one or both of these as a gift.
Cool Gadget for Container Gardening - Sucalc Sunlight Calculator
Container Gardening Gift Certificates
There are lots of places to buy container gardening supplies, plants and gifts online. The following is a list of a few of my favorites - all will do gift certificates.