I have a 4 & a 7 year old. I find it hard to be a "concrete guidelines" person with kids. Staying flexible preserves my sanity.
Our life sounds a lot like yours. At least 75% of our day is filled with activities. On the weekends, I do tend to allow the kids some computer time in the mornings if I am lounging in bed myself. We don't have cable, so computer time to me is equivalent to Saturday morning cartoons. In the evening, I always make sure they are completely ready for bed before I allow them any electronic time, and I always make sure they know exactly how long they have. This is only if they have been good during the day and deserve some winding down time. If there is any misbehavior, my first punishment is to usually take away computer time for the rest of the day or the following day. If there is any whining about turning off electronics, their next electronic time is taken away. They are both (finally) at the point where there is no more whining when I tell them it is time to turn things off.
A key strategy that really seems to influence good behavior in my kids is casually repeating our plans for the next few hours throughout the day. Maybe setting expectations somehow prepares them more mentally to accept what is going to happen. It's a lot of "Okay, guys, this is going to be a fun day. First we all need to brush teeth, pick out clothes, eat breakfast. Then we are going to go to the library on our way to <insert activity>." Throughout the day, usually in the car, I keep telling them how much longer we have at something, and where we are going next, and when we will be going home. Afterwards, "Phew, it's nice to be home. Please put away all your things while I get dinner ready. If you guys are hungry now, you can go straight to dinner, or if you want, you can take a shower first. After dinner, brush teeth, shower if you haven't already, and then you can play 30 minutes on the computer if we still have time." And then I warn them at 10 minutes and 5 minutes left. That way, if they drag at dinner, I can always take the computer away. I find that if I give them electronic time before meals, they forget they are hungry, which makes them cranky, and trouble ensues. I usually don't allow them any computer time in the afternoon, except on the rare occasion I feel inclined to take a nap. After ruling out the computer, their next request is usually a board game, biking, or they do a craft on their own. I have to say, now that they can open the fridge and go to the bathroom on their own, I feel like my job is pretty much done. It's great to have them turning into reasonable human beings. The baby (post 6-months) and toddler ages were much more difficult for me.