The spa thing is a good idea in theory however so many people either don't take the time to pamper themselves for whatever reason OR they go to a specific place and unless the certificate is at that location it might be wasted. (long sentence!).
Some of the things that my clients have given me I won't ever use. Some things I wish I had multiples of!
I don't think you can go wrong with gift cards to local eateries, pet supply stores, video stores, Amazon.com or even local office supply stores.
In this day and age with the economy the way it is... it is always nice to be able to utilize gift certificates to places that you already frequent, hence lightening your load a bit (money going out!).
I own a pet sitting company and I love the fact that clients remember their pet sitter at Christmas. I especially love when I get gift cards to a local pet store. I can head in there or online for the after holiday sales and replenish things that I use in my business (collars, leashes, poop bags, etc...).
I know it seems overly practical on my part but I've never been one to splurge much on myself! Nice to get something that I will actually use. I would then have money that otherwise I would have spent on those practical things to do with however I wanted!
Just my 2 cents.