There are plenty of mechanics type apps available if you have an Ithing. We like Monster physics, sprinkle, where's my water, tinker and oh gosh there is more.
We also like the coding apps. Engineers these days are often needing codeing knowledge so try some of those too.
For toys Lego is your number one choice. Not technics yet. Honestly technics frustrated little kids so save it for later. Normal Lego has a lot of benefits and is worth the expense. Then we liked Magna tiles and pretty much anything magnetic actually.
I often stopped by the vacuum repair place or the small good repair place and got some broken parts for my kids to pull apart using screwdrivers etc as well. My son LOVED this. He took a vacuum cleaning part to kindy for show and tell. Only time all year he was keen to do show and tell!
We have snap circuits but my boy quickly surpassed them and thankfully my father is an electronic technician and my father in law gave him a bread board a little too young...well that's what I thought until my dad came over and showed him what it does and well.......apparently he wasn't too young for it! Clearly I am still too young for it. It makes NO sense to me! So don't be shy to go up age brackets. We have our eye on some of the solar power kits you can get for Christmas