I sent away for a Grinch Winter toque and gloves with grinchy fingers. I was an adult and the gloves were too small, but I wore the hat. 
I got, when I was too young to mail off the application without help from Dad, a little battery powered boat with an electric Airplane Propeller in the “pusher” configuration, like a swamp boat. When put in the bathtub, it promptly turned upside down.
I imagine that Dad had to pay something as well as submit the boxtop: even if it was free, we were ripped off.
I did the same. Then I wrote an angry 8-year-old-penned letter because I didn’t get my first pack and they wound up sending me two. In their defense, my impatient child brain was probably overly liberal with its definition of “8-10 weeks”.
I should have saved both of them so I could today be a bajillionaire but playing with that tiny grappling hook was too much fun.
We were poor, but at least we were never forced to eat the box.
We weren’t allowed breakfast cereal, as our mom believed in a HOT breakfast for her kids, but our dear Granny (she lived several hours away) was a cereal fan, and surprised us now and then by sending away for something and having it mailed to our house.
The first one I remember was a Trix pin (in the affirmative) as seen in this picture
http://www.ebay.com/itm/General-Mills-Cereal-Advertising-Trix-Rabbit-Clip-On-Pin-/330903374836
and my brother received one as a representative of the “anti” crowd.
Another time we each got an Alph-Bits terrarium like in this video
There were several others that I don’t remember.
I think we were allowed to send away for junk out of the back of comic books, ourselves.
I munched my way through a lot of cereal to get a free alarm clock. It lasted a long time though. this would have been in the middle 1960’s.
The first thing I (really my parents did it for me) ever sent off for was a Batman stamp and pad kit. When it came, the stamps were tiny and the pad was dried out, but they were still neat.
We drank a lot of Kool-Aid at my house and sent in the tops of the envelopes for canteens and thermos’ which were crappy because they leaked.
But the best for me was instant gratification when they had the cardboard 45 record on the back of the box they you carefully cut out. Pop out the spindle hole in the middle and in seconds we’re dancing’ to “Sugar Sugar” by the Archies. I don’t think Kelloggs marketing would allow that song on one of their 45 records anymore unless they changed it to “Half the Sugar”.
I got a really awesome Cheerios t-shirt from a boxtop and I think 5 bucks.
In the late 60’s, my mom was REALLY into the space program and forced us to drink tons of Tang, the nasty fake orange juice. For our effort, I got a Nasa commandmodule with a crappy telescope in it and a pretty neat lunar rover.
I don’t recall which cereal, but at the time there was a cartoon about a Snidely Whiplash like character with a snickering dog who flew some kind of airplane race and one of the cereals gave out little plastic airplanes.
The best though was Capt Crunch. In 68, I got this very cool white with blue racing stripes Ford Mustang with front wheels that turned. That was super cool and I wish 40 years later that I still had it.
I had one of these! I can remember my grandpa playing it for me in his workshop in the garage! 
I used to drink Quik brand chocolate milk. They had some sort of a mail-in offer where you could send a number of UPCs and get a Quik Bunny Mug. I was 9 or 10 years old and gathered the necessary stuff and sent the letter off, thinking I would have my mug in a day or two. Days and weeks went by, it was driving me crazy checking the mail each day. Months went by, I gave up. After about a year it showed up in the mail, I had forgotten all about it.
That’s how I got my wife. She’s GRRRRREAT!
When I was a little girl my grandad used to eat Weetabix - every day. Suffice to say he got through a lot of them. He and I had all the bright yellow t-shirts, bowls, spoons, cups, badges, figures, ‘back to school’ stationary sets…
I still have my world Atlas. I loved it then but it’s still fascinating to look at now to see how the world has changed in the last thirty years.
My over-riding memory of my Gramps is of him in his yellow Bixie T-Shirt meeting me from school.
That was the joy of being an only child - I always got the prize in the box. 
For a while (must’ve been '62 or '63) ALL the Post cereals had those inside. Once we trained my mom to look for the FREE U-BOAT! blurb on the package, we amassed a flotilla.
::sigh:: One of my first memories is back in that first little house, up before the parents and watching Mighty Mouse in B&W, and pulling one of those submarines out of a sticky mass of SUGAR Crisp (no “Honey” or “Golden” Crisp – we were honest back then!)…
Sure, as a kid, I sent in box tops all the time.
99% of the stuff I got back was junk… but I got a pretty nice magician set fromWheat Chex cereal.