This is a great question and I'm excited to hear what other Mom's have to say. I have twins that just turned 3 and I am ALWAYS worrying that I'm not doing enough for them during the days.
A good friend once told me that kids at this age should be all about "play". We kind of get told by society that they should be learning and stimulated and structured all the time, but in reality what is best for them right now is to play, play and play.
That being said, here is a quick run down of our day with 3 year old twins and 1 year old boy.
Wake-up: They all wake up at different times. We let them watch Noggin until all the rest of the house is up and going. Probably not the best, but it works for us and they are happy.
Playtime (while I clean up)
Get dressed, brush teeth, Mommy showers, etc.
Play a little more
Baby goes down for a nap; Some kind of art project or focused game, puzzle, books, etc.
Playtime while I clean up lunch
Help Mommy with a project - could be cleaning something in the house, working on a craft, running an errand or whatever is on the agenda that day.
Baby down for second nap; Quiet time for twins (which right now is a movie because we've been phasing out the nap and it's the only thing that keeps them in a quiet mode for an extended period of time. we'll phase it out as they get older)
Outside play or if it's raining then another type of focused playtime project: puzzles, books, games
Family time playing together with Daddy and Mommy
Wind down for bed!
PHEW. So you can see it's not always about structured play at our house. Kids need to be free to build their imagination and creativity and independence with playtime. Hope that helps! Good luck!