G.G.
I did not register for birthday gifts, although many of my friends have done it. On the one hand, it does give guests a good idea what to shop for. On the other, many people will have gift ideas you may not have thought of. My suggestion would be to register, but perhaps not mention the registry to those who you fear would think it's in bad taste.
The length of the party depends on what you're planning. Are parents invited? is it a large, family event or will everyone be dropping their children off? I would say 2-3 hours is the max. Less time if it's only small children.
I'm jealous-- I would LOVE to throw a fall themed birthday party. In October, you have so many fall activities to choose from. What about a pumpkin patch? Many of those offer party packages. Maybe your local park district has a field house in case of bad weather and then you could have activities such as "build your own scarecrow" or "decorate a pumpkin" or "make your own carmel apple" (these items would then serve as your party favors. You supply the pumpkins and paints or foam stickers for pumpkins or you could supply the scarecrow stuffing and ask guests to bring clothing for their own scarecrow, etc.)
Weather permitting, you could have your own pumpkin patch by purchasing a dozen or so and scattering them outside for each child to choose from.
I would check Oriental Trading.com for lots of reasonably priced party favors.
Brookfield Zoo is another interesting alternative, albeit a pricey one (about $35 per person).
I have made this suggestion to lots of Mamasource moms, but I have purchased several ceramic pumpkins from a site called ceramicscraze.com. The owner can make the pumpkin with any name carved into the mouth. It's a wonderful keepsake for many Halloweens to come. Depending on size, cost between $40-$80. Best of luck and enjoy this wonderful milestone.