Play dough and balloons! Cheap and will entertain for hours! Of course, there is a downside... playdough will be messy. Are you ok with the crumbs getting swept into your yard afterwards? Set up an activity station or three, where kids can roam and do what they like to do... Have a plastic tablecloth and set up play dough stuff at one station. Blow up balloons and let each kid have one to toss around and chase, or make a game of it. (do NOT go the water balloon route, it is so messy and can be the cause of someone getting hurt) The obstacle course is a great idea! Bubbles are another fun thing for that age.
Keeping 12 six year olds on the same page for three hours could be challenging. I hope you will have some other adults there to assist. Paper Jet building is another cheap but fun activity. Have the basics figured out in advance, have some adults who can assist and instruct, and then once everyone gets their jets built, and decorated if you want to take more time, line them up and let 'em fly! Have a fly off, maybe everyone who got their plane across this line goes again and so on. or just let them keep throwing them around the yard. You will have some kids who are like Tigger and just keep going and others who will want some quieter activities. So being able to keep everyone happy is your mission. Flarp or silly putty are popular with this age. Maybe have some of this on hand. Bring out the legos and have them in one area to play with.
I will say that while we have done plenty of pinatas, they ARENT cheap to do... because they cost about $15 themselves, then you need to fill it! That is the spendy part. The cheapest filler is really dumb stuff that you may as well not even buy. The stuff that is more fun, adds up. To avoid it becoming a complete sugar mania, we use fruit roll-ups or fruit snacks, small bubble containers, matchbox cars, packs of sugar free gum, glow sticks and even coins. The fun isnt always for the entire group, because often someone breaks it open before everyone gets a chance to hit it and there are always going to be rougher kids who scramble in and grab gobs of stuff while shyer kids hang back not wanting to get rough for the stuff. I have made everyone hold back and then divvied it out, but that was with a smaller group of girls, and they still wanted to go for it! Not sure if you can hold back the boys!
It may seem silly, but Pin The XXX On The XXX is a good cheap game, that you can buy or make yourself. This is no longer tail on the donkey, but whatever you want it to be! Nose on the clown, trunk on the elephant, monkey onto the palm tree, etc. six year olds are young enough to be into this game, and old enough to handle being blind folded and spun.
Musical chairs is a good one too. No cost to you, just get enough chairs, circle them seats out and bring out the music! The chairs could be part of your obstacle course and then pulled together for the game. And if you felt like winding down near the end, you could use the chairs again for a story time and read to the group, one of your childs favorite stories or two.
You can also use the music for a Red Light Green Light game or let them dance to the music and then when it stops, they have to freeze.
My advice is to get all of these things mapped out on paper beforehand. Determine which you will plan to do and in what order possibly, and what supplies you need for each. Perhaps assign each, or a couple, to your assistants, so that you arent running around trying to pull it all off yourself. This gives the adults in charge a little breather between their sessions, but keep the kids moving from one thing to the next. If something isnt working, dont stress - just move on. I assume at least a couple parents will stay? Recruit them!
Not sure what your cake scenerio is - but cupcakes are very acceptable nowadays, you could bake them yourself for minimal cost, then make up butter cream frosting and divide it into sandwich bags and let the kids decorate their own cupcakes before eating them! They LOVE this. have a couple bowls of options for decorating, mini m&m's, choc chips, sprinkles, etc. Put a sheet or plastic table cover down, set up some plastic knifes for spreading and clip the bottom corner of the frosting baggies so they can pipe it out. You and your birthday child could decorate a couple in advance for "examples".
Make sure someone is dedicated to being the photographer. They need to be sure they get photos of the different activities happening and also, should get a photo of each guest with the birthday child. I like to send these in my thank you cards.
And I guess I will end by stating again.... You Must Have Help! And friends, family and other parents should be glad to pitch in and help, if you just let them know what your needs are and where they can assist. Show them your list and ask if they have a particular thing they want to be in charge of. If you do not have a good camera, ask someone who does to come be your photographer (that is a full time gig!) and maybe you have a baker friend who will be in charge of the cupcake project.
I LOVE parties and am very excited for you to have a fun event and make some memories! You Can Do It!