C.S.
1st bdays are for adults, so think about what you would want to do, keep it small so he doesn't get overwhelmed.
With the gifts, I recommend opening them later after the party a couple at a time over a few days. Take pics of each as he opens it and has time to enjoy it, and use those as thank you notes. Opening them all at once at the party is awkward for everyone, especially baby, they don't know what's going on...