1) One thing I would suggest is to get some pictures of him and his life together and make a collage and put it up for people to see, or put some pictures of his life accomplishments around the party, even blow them up, for him and other people to see.
2) Also, have a poster board or book that is decorated so that people at the party can write on it, which says what they love about him so much, then he can keep this and cherish it, you can even put pictures of the party in it afterwards.
3) For drinks have the usual but also make a punch that tastes good but is unique and name it after him, if he is a little sour make it a little sour, if he is super sweet make it super sweet, if he is rugged give it a little rugged taste. Just try some different ones first and pick one that tastes good, then serve it and call it Grandpa John Juice or something like that
4) Food do a old fashioned BBQ if he likes country, YUM, corn, ribs, chicken, etc.
5) A good game would be to make a list of question about your grandpa in advance, such as where was his first job? what is his favorite color? what is the craziest thing he has ever done? how old was he when he got married? random facts or funny things then print out/copy a bunch and bring them, hand one out to every guest or put them on the tables and people can do them seperate or in groups, then everyone hands them in and you read the best answers out loud along with the right answers. Or you can have people answer out loud and whoever gives the best answers wins a prize (something like a box of chocolates or something). It will be fun to learn more about your Grandpa and to hear what other people think.
6) theme could be the year he was born, so things that were popular then or inventions that happened that year, etc.