Z - that is an absolutely fantastic idea!
One thing I've learned about birthday parties (and we've had 5 + 2 of them by now) is that the less you plan, the better, sometimes. We always do a pinata, which we all love, and that's our big event. Until this last year (when H turned 5), that was our only activity, before that, we just played in the yard and hit the pinata and had cake. This last year, we did a little pin-the-badge-on-the-firedog game, pinata and cake.
Trying to corral the kids to actually DO an activity/game takes plenty of time, and before you know it, it's been 3 hours! Don't overplan. If you do crafts, set up a free-play area, where it's simple and takes NO instruction. If possible, enlist plenty of adults to help with direction and corralling of the kids, otherwise, you will miss the whole thing because you'll be running the party and not enjoying it. Simple, simple, simple.
People are coming to celebrate your son, to see each other and have a relaxing time, they're not coming for the groundbreaking games and amazing food and heirloom quality toys in the goodie bag, right? So, just try to stay focused on keeping it simple and focusing on your kiddo.