My daughter is turning two in a week. Right now, she's really into things that you can look at, read, or move.
~Books are always good. Even books that THEY can't read but can be read to them at bedtime or whatever. Or flash cards are nice too.
~Crayons & coloring supplies, Aquadoodle, Magnadoodle, other such art supplies that aren't too complicated.
~"Pretend" items are good as they are just figuring out how to pretend things. A tool set might be a good one for a boy.
~Classic toys such as blocks, cars (age appropriate of course), buckets & shovels for the sand in the summer, etc.
Hope that gave you some ideas. If all else fails, just go to toys r us and walk around. See what the other people with younger kids are picking up.
EDIT: I was just reading the other responses and REALLY like the idea of the superhero capes... maybe you could get them (especially the 4 year old!) some cool superman or spiderman pj's? A friend of mine has a 3 year old that wants to wear his every night!