Duplo Pattern Stacks
I already had the blank index cards, so they were free for me to make. Snag a variety of Duplos from your kids' stash (or just look under the couch) and make several different patterns with markers and crayons. If you don't have index cards, you can get these for .98
Paint Chip Letter Matching
These have come in SO handy for my 6yo who needed to work on letters AND fine-motor skills. You have her take them all apart. Then match the Big letters and little letters and clip the clothes pin onto the Bigger paint chip. She can turn it over to see if the colors match so she'll know every time if she's right.
The paint chips were free at Lowe's and I already had clothes pins. Here are some different quantity options for you: 24 for $1 or 50 for $6
I glued them onto paper, but wish that I had used cardstock or index cards.
NOTE: Make sure you get really distinctive paint colors to avoid confusing your kiddo.
Carland
Does anyone with little kids NOT have any matchbox cars? And cardboard boxes from Amazon? Whammo! Free busy bag. Cut blank sections of cardboard off boxes into squares that will fit into a gallon ziptop bag. Draw different kids of roads or a parking lot, etc on it. Let your kid rearrange them to their heart's content on the floor and drive cars all over it.
Eggs and Poms
So the eggs have numbers and dots on the different halves written with a Sharpie. Your older kids can count the dots to find the right other half. And put that many poms in the egg. And even match colors of poms to eggs. Your younger kids can just open the eggs and stuff poms in there. Tons of fun! And all of it fits in a quart or gallon ziptop bag! Eggs Poms
Buttons
I bought this one also, but it would take ~5 minutes to make. Your kid just dumps out the buttons, then one by one, drops them through the hole. While I type this, it sounds really boring, but my 2yo LOVES it. Make sure you cut a Big enough hole for the largest buttons you give your kid. The ones we have are like this. And you need lidded small bowls like this.
Macaroni
They have a blast scooping and pouring the dry macaroni! This cost: $0. I had leftover macaroni (you only need 1-2 cups), a couple Bigger bowls that I didn't use anymore and some unpopular measuring spoons (seriously, donate ones you HATE to this busybag and buy some of these--you're welcome!)
Little Wooden Puzzles
Ours fit tidily in a sandwich ziptop bag and our kids like being able to feel all the different pieces (because they're wood, not cardboard, so they actually have texture). They also only have like 3-4 pieces, so they're easier for them to do and enjoy that accomplishment! I couldn't find the exact ones we have, but these, and these are similar.
Princesses
My girls LOVE these! I'm pretty sure they came from Walmart's checkout line, one princess at a time for a couple bucks each. They have changeable dresses, which of course get changed CONSTANTLY. They easily fit in an old shoebox for pretty convenient storage.
Magnetic Dolls
These even come with a super convenient carrying case. So much dressing up to do!
Minions
They easily fit in a sandwich baggie and provide lots of fun play time
Disney Princess Enchanted Cupcake Party Games
This is an actual game. That my kids never wanted to play as a game. They just like to make the cupcakes. So it's just become a busy bag. Pick your battles, #amiright?
Brain Flakes
This has tons of pieces so LOTS of kids can play and build to their hearts-content (but beware a younger sibling dumping them out--eek!).
ALSO...
We have an Activity Shelf (a bookcase that houses all the kids' craft stuff and activities) and the busybags weren't fitting well, so I used small command hooks and my trusty hole-puncher to have a tidy space for them on the side.