Wargaming Miscellany has an article on building a hollow hull for a larger-sized model ship.
Modeling
Making A Sheet Plastic Vacuum Former
With your own homemade sheet plastic vacuum former, you can (easily?) reproduce lots of different things for your wargames table, especially terrain features.
Building A Ruined Tower For $1
This video shows a neat build of a ruined tower terrain piece for your wargames table. It would work well for all sorts of genres and periods.
Making A Stone Barn
Barking Irons has a photo tutorial on making a stone barn for 28mm figures.
Making “Vertical” Hills From Wood Chips
Maiwand Day has a nice photo essay on making a wargames mountain with a vertical profile. It’s really great looking, and is on my list of future projects