I have done a lot of different birthday parties. Until recently, I would do the planning of games, with my daughter's choice of theme. Her first birthday was pretty easy, because we just made a cake and invited a few friends. Since then, I've done a Dinosaur, Magical, Zebra, Tiger, Minecraft, Wolf, Spyro, Horse, Horse/Dinosaur, Polar, and Warrior Cats themed parties. Most of my daughters friends don't have a themed party, so it's fun for everyone. If you have a theme with games planned it keeps them occupied. It also keeps the gang from destroying the house and your kid's bedroom!
Start the party between lunch and dinner so you can focus on the fun and make things less stressful. My daughter's birthday is in the winter. We have a decent hill in our yard, so we begin with sledding as the kids arrive. The sleds are usually out. I ask them to come with their snow gear on and bring their own sled if they have one. This gives you time to get the last few things done, maybe take a few pictures and get the treat bags ready. We do a scavenger hunt every year for the treat bags. It is based on whatever the theme is,4 or 5 clues are plenty. Make it more difficult as they get older but not so difficult they can't figure it out. When the kids were little it was often inside, as they got older and louder it went outside.
Once inside, they are usually ready for a slower pace. Never underestimate the Pin the ----on the ---! I am pretty decent at drawing, so we usually draw up a picture of the themed item and have guests pin on a tail, horn, mouth or the item. The next game is the creative one. For the Tiger theme, we did Smellovision: Blind fold the guest and have them smell different things in a box or paper bag, see if they can guess what they are. While we were still outside I had looked up how far different Big cats could jump. The kids took turns seeing how far they could jump, and how many it took to get to the snow leopard's (they jump the farthest) For the zebra party, I put different safari animals in a bag to see if they could guess them. At the Polar party, I pre made snow animals outside, and had them work in teams to find items for the face and arms and dress them with a scarf or hat. For the Horse theme, we had paper horses from a shaped note pad. We raced them by blowing them across a finish line.
As far as decorations, keep it simple! Get streamers and/or balloons that go with the theme. Buy a disposable table cloth and add your own pictures from the internet to it by taping them on. Decorate around the house the same way, or use stuffed animals and toys. Make the treat bags together with the birthday kid and let them design their own with a lunch bag. The birthday kid likes getting a treat bag too, so don't leave them out. Think striped candy and do dads for tigers and zebras; blue, white, purple items or wrapped candy for polar; basic animal colored items or go to Oriental trading if you're stumped. It's a party after all, don't stress yourself out!
After the games, it's time for cake, ice cream, and opening gifts. Don't forget to make a list of who gave what so you can teach your kid the importance of a thank you card. Have them mail it by post, so they learn how to address it too. All together plan the party for 2 to 3 hrs long. Keep things moving so you cover everything. Don't give out prizes for winning individual games, everyone gets a treat bag. Plus, it's less work and hopefully no crying kids. I tried to attach a picture of the latest cake, but it took up the whole writing area. I hope this helps you plan a fun birthday! Enjoy!