I noticed no one mentioned outdoor temperatures. When my ducklings were two weeks old, it was 38 degrees F outside. They stayed indoors, as the recommendations from Storey's Guide to Raising Ducks are to keep them around 85 degrees their second week, dropping to about 80 their third week.
Last weekend, we had unseasonably warm (near 70 and sunny) weather and then I had them out for a few hours in the afternoon (now about four weeks old). They won't have enough feathers to keep them warm at night (night before last it was 25) for a while yet. They also still need to put on a little more flesh, so their surface to volume ratio will help prevent hypothermia.
For treats, there is a stickie on the duck forum here from Nettie, who has compiled recommendations for treats. Some things need to wait, some things are safe for younger ducklings.
I give mine finely shredded salad (lettuce, cress, mache, dandelion, chard) almost daily (2 or 3 cups for 11 ducklings) and thawed frozen petit peas (about 3/4 cup for 11 ducklings) every couple of days.