Separation is never easy, particularly with such a young child. I've heard of people making a big print of their soldier's face & putting it on a stick so they can insert him into their adventures, take pics & send them to him so he can see you carry him with you everywhere you go! It helps keep him near for your daughter as well. Invest in webcams so he can talk directly to your daughter and you. They have helped many a family during this type of separation.
As to a gift to send, it's hard to know, my son is deploying this winter, too. I keep thinking of making a quilt but he will be in the desert so that'd be a waste for him. I think you should sleep with whatever you send with him so it has your scent, same with something that is your daughter's. Scent is a powerful thing. I'd guess it probably should be on the smaller side so he can carry it. Maybe make a pretty heart or something so he can have your heart with him. It's corny but hey, this isn't ordinary circumstances!
For your going away party, maybe a simple meal with all his favorite people & foods. Keep it simple and do the meal a week before he goes so you can have him to yourself the last few day, maybe the night before have his parents there, too if they're around.
Thanks for enduring this hardship for our country, I'm a veteran and an Air Force mom. I know this isn't easy or what some people would chose, so I thank you for being strong for all of us. If I can help in any way, drop me a line. I mean it.