I'm going with music rather than a second spoken language for my kids. It's something I do actively that I believe will be similar in benefits to the kids learning and cognitive development.
I've started with piano (keyboard connected to a computer actually, I have thousands of sounds it can produce, they like the variety) and then, in a couple of years (5-6 years old) moving to guitar (I play) or violin (my wife plays), depending on what they each want to do. I'm also learning to play piano more as well (I previously only knew chord patterns).
We also have a variety of kids and adult drums/bongos (love my djumba) that we bang around on a lot (great for camping).
Why not a second language? It will be unused and quickly forgotten. So music is the language of my choice and I hope I can pass on a love for music as my mother did for me (well, she had an electric guitar laying around and I just started playing it, but having it available was the critical part).