I think that's really true that lots of children don't 'just play'. I definitely try and make sure there is time for just mucking round the house and garden, and as for mud, it is a very important part of dd's life! She is never happier than when in a pile of mud in her 'booties' as she calls her wellies. But some of my friends' children are very over-timetabled imo.
Our timetable (term time, holidays are different)
Monday Am - Ds1 School, Dd Playgroup/Activity Ds2 Activity
PM - DS1 School, Dd play at home, Ds2 nap
After school - go to friend's house
Tuesday Am - Ds1 School, Dd Gymnastics, Ds2 gymnastics
PM - As Monday
After school - Ds1 Gymnastics, Dd and Ds2 play in the foyer at gym!
Wednesday AM Ds1 School, Dd Playgroup or Swimming, Ds2 Swimming
PM As Monday
After School - Ds1 did have after school club, but from now to the end of term we're going to do things like going to the zoo!
Thursday AM Ds1 School, Dd Playgroup, Ds1 play at home
PM As Monday
After school - Playdate usually
Friday Am As Thursday
PM As Thursday
After school - play at home
Saturday AM - Swimming - then free for the rest of the day
Sunday AM - church, then free