My morning schoolchildren (boys and girls) tend to amuse themselves as I'm usually doing breakfast for babies - they sometimes watch TV/video, use games console, or play with anything available!
After school, I tend not to be too regimented about things as they usually want to just chill out. They often like mucking in and playing with the younger children and their toys.
They love any construction sets (Lego, Polydron, anything with small pieces), but also love Playdoh, painting, drawing, glueing (even though they say they don't!) If I'm likely to be really busy with little ones, I'll put stuff out on the kitchen table for them to help themselves to - lots of paper, scissors, pens, stencils etc.
We have a small snooker table, football table, board games, puzzle books, marble run, books, cars/trains/tracks, Twister, dominoes - the list is endless. Get them to write a list of what they like (that should keep them quietly active for a while).
You haven't taken on too much, it will just take some getting used to! It tends to be much easier when the weather is nicer as they like being in the garden.