How I love a carousel, however, unless there is one in your area, you probably aren't going to be having one for your child's birthday. I'm not going to recommend going all out and hiring bouncing castles in your backyard, or petting zoos, clowns, and the like. I am all about simple fun that is achievable and memorable.
Do you realize how easy it is to make a maze out of haybales or cartons? Really easy and fun, if you are using cartons, you could get your children to paint them beforehand so you have a colourful and zany maze, it doesn't have to be too big, just enough to be interesting and don't forget to have something fun in the centre, perhaps a table of toffee apples, or cupcakes.
If you don't want to spend a small fortune booking a petting zoo, you could still use any connections you have in the local area, perhaps someone with a pet donkey, or a pony who will come to give rides for your guests.
Before eating, get the children to sit at the tables you have laid out to make their own party hats. If you have old christmas cracker crowns or pre cut your own crown designs, the children can apply pictures, ribbons, sequins, buttons, feathers, or stick on gems to their heart's content using simple paste. You might need another parent to help make this run smoothly but I think it is a good group activity as no one wins a prize, everyone gets to make their own hat that they can take home.
I bet you grew up with pin the tail on the donkey, pass the parcel, and musical chairs, so I don't need to explain them. I do find they can cause some squabbling, and not everyone can win.... so I like things like treasure hunts, where each child is only looking for their coloured counters or marbles, which are then cashed in for a set amount of booty - ie, no one child can get the lion share of treats, while others miss out, each child can only get a set amount. If one child is having trouble looking they can always ask for help, but not at the expense of their treasure, they still get all their treats.
You could try a caterpillar, snail, or frog race for a boys party (or girls if they are that way inclined) you only need to mark the number of lanes, you could do this with coloured yarn, texta, or paint depending on the race surface.
For girls you could perhaps have nail polish manicures (depending on age), or temporary hair colour sprays (ask other parents before the party if they mind their child having punky hair streaks for a day or two).
I still have the photo of the human pyramid we made for my 13th birthday, and of course, posing for photos is a must, and will use up a nice chunk of time.
I love bubbles, so you could let the kids go to town with bubble blowing out in the garden, another great photo opportunity.
If you have karaoke or playstation singstar, girls seem to like this a lot, and it is good, safe fun. Dance lessons are a good activity too, you could book a hip hop dance instructor for an hour, or buy an instructional dvd (if your living room is big enough).
Pinatas are easily made using crepe paper or you can buy premade ones at Spotlight, Kmart, and other stores. Of course the birthday child gets to crack it open but you can put the party favour bags for all the children inside, with their names printed on them, so no one misses out.
I hope these ideas have inspired you, I will likely have another post like this in the future as new trends come along. Some ideas are timeless though, and I like to encourage good, safe, simple, fun.