Opening the bag will give you seeds appropriate for your current Farming level. It will also provide some coins.
The following seeds are possible to get from the bag (Note, you won't get all of these seeds, only the ones appropriate to your level);
Vegetables: Potato, Onion, Cabbage, Tomato, Sweetcorn, Strawberry, Watermelon, Snape grass, Sunchoke, or Fly trap.
Trees: Acorn, Willow, Maple, Yew, Magic, or Spirit.
Fruit trees: Apple, Banana, Orange, Curry, Pineapple, Papaya, Palm, or Calquat.
Herbs: Guam, Marrentill, Tarromin, Harralander, Ranarr, Toadflax, Irit, Wergali, Avantoe, Kwuarm, Spandragon, Cadantine, Lantadyme, Dwarf weed, Torstol, or Felstalk.
Bushes: Redberry, Cadavaberry, Dwellberry, Jangerberry, Whiteberry, Poison ivy, Barberry, or Wishing well.