Ask your sibling if it would be ok, to take her out to a museum or something similar.

Or aks them what she currently is into and try and get her something you know she'd love.

I once bought my niece a marble run. I got some shit for it, because I guess some people just don't get it? But she loved it, and my 6-year-old daughter now loves marble runs too.

My daughter also collects rocks, so a rock tumbler was a big one for her.

Another gift for a niece I got shit for was a drum. She loved it. She was so excited that it was a real instrument. My brother always said he'd get me back, but my daughter got a full-ass drum kit for Christmas, and I think it's great.

Oh, and make your own slime kits are huge right now. It's science-y, DIY, and kids love slime.

Sudoku book, maybe? If the likes making things, origami paper and an insteuction book? OH, I remember as a kid this toy that was short plastic sticks with magnets at the end and ball bearings ao you would make structures and stuff with em, fun to play with and suits the vibe you're looking for I think.

I was about that age when I was gifted a microscope. No idea if you can still find them that cheap, though

My 6 year old son absolutely loves the Snap Circuits kits. We've also started teaching him collectable card games (Pokémon, Yu Gi Oh , etc) which challenge his reading and strategy skills. Plus great quality time activity. There are tons of Stem kits out there for less than $40.

Charades for kids. It uses her creativity.

ant farm

Board games. Catan junior, Loopin Chewie, Robot turtles, Tsuro, Abracada...What?, camel up.

Regular price

Spirograph, Crystal growing kit, search for stem toys for 6 year olds, ask her parents what she's into.

I've been doing stem toys for my niece for a few years and she always loves them. She just turned 9 in August.

Modeling clay and good quality paints.

Artistic, technical, no real "rules", and a good amount of skill building.

She needs a sword. It’s educational.

Thinkpad thin client with Gentoo

Is she too young for an Arduino starter kit?

https://store-usa.arduino.cc/products/arduino-starter-kit-multi-language

Edit: the great thing about these is that you can eventually add a raspberry pi to the mix for even more functionality and learning fun!

At about that age, I enjoyed putting together dominos runs and knocking them down. I also liked wooden building blocks. Nowadays they have building made out of dense foam which probably hurt less when your baby brother knocks your tower onto your head.

Get her a rock tumbler.

There are also kits for learning about electricity for kids of that age. For making a light switch or making a doorbell buzzer and simple things like that.

Maybe not in the "smart mind challenging" category but there are plenty of craft kits for your budget: paint canvases by numbers, make your own accessories with clay, bead jewelry, basic engineering kits (build your own robot types), dig your own fossil kits, build-this-or-that-with-LEDlights, gardening kits for kids, etc.

My point is to expand your horizon a bit, it's perfectly fine that you want to cater to her intelligence and not just go for something girly in a cliche sense. But she can also enjoy crafting stuff.

Get her started on the "exclusive club" membership scam. She makes a "Super Secret Smart Kids Only Club" in the backyard, charges neighborhood kids $5 to join, promises amazing activities and snacks. First meeting is just them sitting in a circle while she explains the "rules" for 10 minutes, hands out one Oreo each, then says they need to pay another $3 for "premium membership" to unlock the good stuff. The good stuff never materializes but somehow kids keep showing up... What? Oh.. GIFT, I thought you said... Nevermind.