First of all, Congratulations!
I don't know if my advice is good, because I'm really shy and non-date in the know, but after more than a yr together w/ my first b/f, I invited him to my birthday party, it was just my family and him. We went to Sea World, w/ he and I venturing off ourself. He really enjoyed it.
We broke up, but got back together a few months before Easter. He surprised me by presenting me with an adorable pen shaped like a bird. Before school got out, he just randomly gave me a bracelet he made, (and he made one for himself just like it). The smallest things are also the sweetest things. dont' go overboard and stress too much. I'm sure you'll think of something that he'll remember forever.
surprise him with something sweet...like call him to caome over to your house and when he gets there have his fav food cooked, his fav movie, just make it a night for him and just spend time together
te-he for the first month i got my boyfriend a blue teddy bear that is small, since he like blue things and he like cute things [he called me cute might as well get something cute for him]. second month i got him a love heart box with stars in it. i made the stars myself and sort it by colour bah i was lame~! thrid month i can't get him anything because he told me off for buying him things ==" so i'm gonna give my third month present as my fourth month present. the only reason im skiping third month and go for fourth month is because on the fifth month i won't be able to be with him cause i have to go overseas. sigh. oh the third/fourth/fifth month present is a sand box thing and when you flip it around it says "i love you" and the sand colour is pink bah! i love pink~ te-he =D well yer i hope that helped?
He's already cheated on you with your best friend, and some chick who works cashier at McDonalds. You may as well give him some great top or else he might leave you soon.