Things like a magnadoodle or aqua draw (art with less mess), board books they can look at with out you having to turn the pages, simple puzzles, push along cars/trains, a baby doll to parent/nurse, a selection of long shelf life snacks like mini breadsticks, raisins, fruit puree pouches, wooden spoon/tupperware tub for a drum (much quieter than a real one you can put rice/dried beans in too, to make a shaker), stickers & paper (wrapping paper is good too for scrunching/tearing), or a shape sorter are all good things for a nursing box.
If you get a big floor mat rather than the small table mats/character scenes your darling children can turn it into a paddling pool by emptying their cups of water over it They are truly multi-functional
The wands/pens are good as they open the wrong way (screw cap turns the other way to normal) so it's difficult for children to open if they are used to taps/lids/etc that open the other way. Not so great for children who can't yet reach the taps so don't have the instinct to open them clockwise and empty them first try <grrr>
Also, don't leave out pens/crayons at the same time though as they will colour the mat in, I promise.
Trains, cars, tracks/roads for them. I bought an absolute huge load of this stuff - sometimes it's great when they watch a Thomas DVD and you have the actual characters to play with/recreate the story. DS was 24 months when DD was born and we had no problems whatsoever (I bfed DD for a year, but was not tandem feeding DS as well). Also some movies - Toy Story etc.