I taught Jukes this one a few months ago
he already had a solid retrieve and knew that 'toys' meant every toy, so I just got him to fetch the toys and then like the posts above, put my hand over the box and got him to drop it whilst saying 'tidy'- then immediately rewarded with a treat (I have one food crazy dog)
Over time I slowly got rid of my hand until he just dropped it in when I gave the command 'Tidy'
How's your dog doing with this? - it's one of Juke's most useful tricks