I.M.
Hi M.,
What a wonderful thing for you to do. My son is nearly 6, and for the past 2-3 years, has been obsessed with any type of building toy like the wooden erector/construction set from Melissa & Doug and the kid-k'nex sets from K'Nex. These have large parts which snap together easily and can be used to make anything from cars to critters to whatever his imagination dreams up.
I think the most important thing for this age is to select toys that can be used in multiple ways; that is, not just a puzzle that only goes together in one way, or a car that is only a car. Building sets help kids develop both their fine motor skills (to put the parts together) and their visual/spatial cognitive abilities (what parts do I need to make the airplane I'm imagining).
Hope this is useful!
Happy Holidays.