K.H.
Coloring is a valuable exercise in identifying colors. Go on walks and talk about the colors you see. When you are grocery shopping talk about the different colors of the fruits and vegetables. Stuff like that. It doesn't have to be structured, it just has to be informative.
Storytime can be more interactive. Instead of reading the same story over and over, ask them to identify new things on page. Have your 3.5 year old make up the story.
For the alphabet, while you are preparing a meal. Explain A is for apple(or whatever it is you are preparing). Later, have them go around the house and find items that start with a particular letter(find all the items that start with D...something like that)
I have started my 2 year old with puzzles. It enhances her matching skills and helps with her fine motor skills(which she has been good with all along)
Why not enroll them in a part-time preschool...