If you're really looking for some spooky, more grown up Halloween decorations, I'd highly recommend visiting some "official" haunted houses and seeing what they have. Many are very simple by having people dress up and pop out to scare people at random spots in the house.
I try every year to get good decoration ideas and I always browse around. Here is a good site that has many great ideas. http://www.hgtv.com/topics/halloween/index.html
There are many Halloween party ideas. Some popular Halloween party ideas incude monster parties, blood-like drinks, costume parties, spooky games, etc.
Spooky halloween decorations would include things such as ghosts and ghouls. Try party city or your local party store to find spooky halloween decorations.
Some great ideas for fun Halloween treats are candy apples coated in chocolate, Rice Krispy squares with pumpkin designs, or homemade spooky cat cookies.
To throw a child's Halloween party, make sure you have some spooky snacks in addition to candy and a special Halloween "prizes" for the best costume. You can also have a pumpkin pinata, or pumpkin cake to serve for dessert.
Some of the specialist Halloween sites offer good lists of ideas, hoping to lure you back to purchase later. Alternatively, various lifestyle sites do occasional posts, for example: http://www.moneycrashers.com/cheap-DIY-Halloween-decoration-ideas/
Some people tend to go with a theme when they decorate their homes for halloween. Like if you wanted to go with a witches den theme, you might pick up a spooky cauldron and work the rest of your decorations around that.
I'd recommend making your own decorations. This can be as simple as buying little pumpkins for all of the kids and letting them paint faces on them. Halloween does not have to be an expensive venture.
Some of the easiest halloween decorations may be black and orange crepe paper that you can twist and string from items in the room. You can also get fake spider webs for doorways and walls.
You can find some awesome patterns for decorating a pumpkin at http://www.marthastewart.com/photogallery/pumpkins. You can also find more ideas at http://www.countryliving.com/crafts/projects/pumpkin-decorating-1009
Scary halloween decoration for your yard would include ghosts, fake hands sticking up out of the ground, and of course a fog machine. These would be horrific.
You can find Halloween party ideas in websites such as www.halloweenpartyideas.org/; www.celebrations.com/halloween-party-ideas; and www.familymanagement.com/holidays/halloween/party.ideas.html