Besides Legos, what’s the best kit for prototyping mechanical devices?

https://www.fischertechnik.de/en

https://en.wikipedia.org/wiki/Fischertechnik

We had those in primary school and I think it was electronically more advanced then Lego was at that time (they have added a CPU unit in 1980). We actually built a caterpillar based vehicle avoiding the obstacles using a photosensor in 8th grade, so this would be 1989/90. At that time Lego was just a toy (or maybe the Lego Technic wasnt available where I was living, I am just looking at wiki and it looks it was introduced in 1977 and they had a motor but no electronic counterparts yet) and I think Fishertechnik was mostly used in schools due to their electronic parts (at least in my country, I dont know for Germany).

https://en.wikipedia.org/wiki/Capsela

https://en.wikipedia.org/wiki/K%27Nex

Capsela and K'Nex were both great. I absolutely loved capsela as a kid. Capsela was an incredibly satisfying toy to play with and it was simple to put together. Relatively small changes created significantly different machines. Understanding the toy made it more fun to play with, but it was fun with almost no understanding too. Playing with it increased understanding.

This is not from experience, but what I expect the answer is for older kids:

A 3d printer with help understanding how to mechanize is probably best.

Adafruit is probably one of the better websites for acquiring interesting parts:

https://www.adafruit.com/

Maybe what you're looking for: https://www.adafruit.com/category/227

It's hard to give a better answer without knowing age/budget/goal/etc.

Not sure about availability around the world, but I would say Merkur: https://en.m.wikipedia.org/wiki/Merkur_(toy)

Fun fact: first line for creating contact lenses was build using this system.

I'd argue Meccano is probably the most famous and storied - if you have the money

Nowadays I'd just get a cheap FDM printer and fasteners, though

I've recently switch back to good old balsa wood. I've been 3d printing for years and metal working for longer. Going back to easily worked wood has in many ways accelerated some developments.

If you're building anything more than toys or tinkering; if you're actually trying to design a product or something functional you want to use, a 3d printer.

I resisted getting a 3d printer for a long long time. This is coming from a man with a fully kitted-out woodworking shop, 2 CNC machines, welding equipment and a lot more.

I didn't see the 3d printer as being for anything besides miniature figurines and cheap tat, because that's 99 percent of what you see people printing. But it's SO useful. Adapters, enclosures, brackets, 3d printers take care of solving all those little niggly bits that are really hard to fabricate by traditional methods.

I recently upgraded to a Bambu X1 Carbon and highly recommend it. I also have a cheap flashforge resin printer which is great.

Two things I've used my printers to make, that have ended up as actual products; a guitar pedal enclosure FDM printed box with an open top, I used aluminium PCB panels from jlcpcb.com as the front panels. The result is a very professional and sleek looking design that's tough as hell. The other one is transparent push buttons for a Eurorack synth module I'm working on. I used my resin printer for this, and it allows me to make a custom button layout that sits on top of mini tact switches with backlights, and gives a user interface similar to custom-made carbon membrane buttons. - similar to https://en.wikipedia.org/wiki/Silicone_rubber_keypad

If you actually want a do it all solution, learning CAD/CAM and 3d printing is the way to go.

You can do a lot of stuff with CAD/CAM before even getting to the 3d printing stage.

Totem looks interesting and cost effective compared to MakerBeam, though obviously less rigid.

https://totemmaker.net/

Foam core, or even plain styrofoam's a good one, though not so ecologically friendly. Simple cardboard and tape can work, but it depends on what you're making, so the question to you is what level of mechanical device are you making, and what resources and knowledge do you have? Because 3D printing is popular and welding isn't that hard, but both of those have not-insignificant startup costs.

For adapting kids toys to this purpose, there's also K'nex.

Vex has quite a few options depending on what you’re building.

https://www.vexrobotics.com/

Quick prototyping with wood is still dang good, because you can put the screw or glue wherever you need.

If you use the right screws you can even add some adjustability.

T-slot, and the better Japanese version the name of which escapes me but I found via HN so I hope someone will fill in.

80/20.

Meccano

A 3d printer.

gobilda has a great library of parts for building solid mechanical devices

https://www.gobilda.com

3d printing has been a complete game changer for me. It's incredible how you can make useful and completely custom parts in just a few hours.

Fischertechnik

Lynxmotion is good for servos.

A 3D printer and aluminium extrusions.

Makerbeam is awesome

K'nex