Simple garage to fit existing cars - a few angled ramps and levels
Simple wooden truck. Make the trailer big enough to carry toy cars, lego models etc. and they will use it more.
I made a rolling trolley that went under the table - wooden brio train track on top angled track ramp down the side and then it can be connected to floor track and built out into big layouts on rainy days. That was a big hit for many years. can be wheeled away out of sight.
Spent a lot of time making marble run channels and bits. Wasn't massively popular though compared to all the effort put in.
Wish I'd just made something like this!
Wooden rocket - shape some wings on bandsaw / sander. Shape a 34mmx34xx piece of timber into a body. Glue the wings on and let the kids paint it!
Water play - hours and hours of fun. Simple table or bench with two levels - old plastic box with a water butt tap fitted on highest bit. Lower down a little platform where the water can flow over - down to a bucket so they fill the top box up over and over.
You can get simple doll house / play set type things where just 2-4 pieces of md or wood planks slot together with doorways and windows cut into them - good skill builder project. Look at pinterest for ideas.
Lots of comments about spending time with your kids. It's a good point. But if you make simple projects in the evening after they are in bed. (Hand tools or a quiet pillar drill won't upset the neighbours) then you get the best of both worlds.
I will try and find some pictures as I've made a few of these.
Good luck
Martin