So many options! I always try to make it personal.
I had some clients who wanted to grow their own garden so I got them a gift basket filled with gardening tools, seeds, and other things of that sort and a baby apple tree for a head start. Otherwise, they would be waiting awfully long for their first apple!
Another client always had to have his morning coffee, so I got them a nice coffee maker for their new kitchen.
If you can't come up with any ideas, gift cards to HomeDepot, Lowes, Target, Crate&Barrel etc work great. Otherwise, you can never go wrong with a thank you/congratulations dinner. Sometimes, your clients will treat as a thank you to YOU for all your hard work. =)
I think the more personal the gift is, the more appreciative and meaningful it will be!
Pinterest has always helped me with creative ideas.