Nix the balloon idea. My ex turned 50 right after we split. He told the kids "no party". Uh - are you kidding? The kids would not have a party - they never even went to his place (EVER) because of OW. I did get him a card and a small gift from Ryan. (He can't do his own shopping). But did he really think we were going to have a party?
You don't like your ex - that's obvious. Doing something to announce it makes YOU look silly. Or mean.