I think music CDs are the best gifts. Can never have too many. I recommend any of the kids' Putamayo CDs and the Beethoven's Wig CDs.
And ask for clothes 12-18 months ahead in size. I really like to get those rubberized socks because they are expensive and you can never have too many different colors.
Also, I think Fisher Price Little People toys are great first toys up through preschool age. ALthough your four year old may have these.
Also check out some unique European toys at sites like Oompa. HABA blocks are fantastic and you can get many different sets - for instance , a basic natural wookd block set now and over the years the fancier architechtural blocks. Alphabet blocks are good too.
Also, think ahead to baby food around 6 months. If you are going to make your own food this time, you want a good blender/food processor and nice ice cube trays. Plus you will need all the bibs, plates, sippy cups (and extra valves), etc. You may have some, but I ended up starting fresh with a lot of stuff since my older child had used everything for so long. Every second child deserves a few new things, right? :)
Personally, I don't see the neeed for Bumbos. My SIL loved hers, but I think you can come to rely on it so that the baby doesn't develop their sitting muscles. YOu have to be careful not to over use it. It also has a very short life. Some people use it as a booster, but they don't attach to anything and ther is no seat belt, and I don't think that's very safe. However,getting a travel booster for taking to Grandma's is a great idea.
Enjoy your Christmas!!