I presume we're talking fairly low budget? Coming so soon after Christmas and all? I'll give you some options OK?
Invitations
Download an image (something like this maybe and superimpose your text onto the top of the pic (or have the pic in the centre of your invitation and the text at the top and the bottom)
You want something like. Invites: "YO HOW YOU DOIN'? THE CHAMP WOULD LIKE TO INVITE YOUS TO CELEBRATE ___'S 30TH BIRTHDAY THIS (insert date) AT (insert time). DON'T BE NO BUM; COME DRESSED IN YOUR BEST "ROCKY BALBOA" ATTIRE & BE READY TO ENJOY A KNOCK-OUT SURPRISE BIRTHDAY PARTY!" (I found this link with loads of ideas for a Rocky party) If you want to make it fancy dress then you can stipulate this on the invite (v easy to do really, just tracksuits or dressing gowns and shorts!
OR you could make it just an 80's fancy dress theme if you want - it'll work!
If you're a little more flush how about here and creating your own invite online? I wonder if you could somehow get your DH to do a Rocky pose and take a pic without arousing suspicion.
Decorations
Some ideas here
Love the idea of making the main party room look a bit like a boxing gym - you can definitely download a bunch of images from the net of stills from the film and blow them up before printing. Rope is a great idea and you can also go for lots of american flags look here or you can go for the 80s theme here
Food.
Right - I would deffo go for a US theme foodwise. Rocky was a Philadelphian so I'm SO tempted to tell you to go for their state food - Philly Cheesesteak sandwiches, but I fear this will drive you potty and is not worth the stress...so instead how about a buffet of traditional american foods?
So for some authentic bits and bobs look here it's a bit expensive so just a few bits I think - a big bowl of "candy" maybe or some Gatorade to drink?
Do you want my pulled pork recipe? You can serve pulled pork sandwiches, southern fried chicken, popcorn, hotdogs and burgers - most of this stuff can be cooked in advance and kept warm.
Music - piece of piss, all 80's obviously.
The SURPRISE - have you thought about this yet? It would be great if as he walks in you could have Eye of The Tiger playing.
Mammoth post.
Tell me if you need anymore...