Write details to go with your picture: Give your "baby" a name, birthdate, and brief profile (e.g., feeding style, sleep habits, any “medical needs” like colic or allergies,etc).

Rice or dried beans: Fill a gallon-sized Ziploc bag with ~7–9 lbs and place securely inside the doll or swaddling blanket.
Sandbags or ankle weights: Wrap in soft cloth and place inside doll clothing or a carrier.
Water bottles: Use two 1-liter bottles (roughly 4.5 lbs each), wrapped in a towel and placed in a bag with the doll. Or a filleed hot water bottle.