Why not let them pick out their own focus fabric so you are sure they will get something they love, and then you can pick the coordinating fabrics and the design which is what will be the surprise when finished.
I did that with my nephews and they absolutely love their quilts. They picked their focus fabric, and if it was really busy, then I used it as the center of a framed block and did 9 patch corner stones and sashing for one that had blocks of vehicles (race cars, racing boats, racing motorcycles). The second was a busy baseball fabric with an American flag design, and I did the same, but the cornerstone was done in stars and the sashing was solids. The third wanted a Boston Bruins quilt, so I purchased 12 embroidered logos from Ebay, I framed them and alternated the blocks with stars, using square cornerstones and solid sashing in gold and black, and all three were quilted with themes from www.quiltrecipes.com.
We looked for focus fabric at www.equilter.com, they picked it out, then I shopped further for the best prices at online stores. They had fun doing that too.
They will know they are getting a quilt, but the finished product is the mystery and a very anticipated one!
Diane