Here's what to pack

For babies and toddlers:

Plenty of diapers, wipes, changing pad
Travel potty seat
Bibs, big napkins
Toys, teethers
Several changes of clothes
Socks and shoes
Child washcloths and baby soaps and shampoos
Undershirts
Sweater
Baby hats (one to shield baby from the sun and one to keep baby's head warm at night, depending on your travel destination and season)
Swim diapers
Baby Tylenol or ibuprofen and dropper for dispensing
Thermometer
Baby's medical record and hospital identification card
Baby sunscreen
Food for the baby: Baby food, formula, juice, etc.
Sippy cups, spoons, baby bowls.
Empty bottles and nipples.
If bottle feeding: bottles, 8 oz. cans of formula (ready-to-eat type), thermos with hot water, big cup in which to warm the bottle
Stroller
Stroller netting and rain shield (depending on destination)
Several blankets
Sleep aids: lullaby tapes, ocean noises, etc.
Nightlight
Educational videos if the hotel has a VCR. (These are helpful to keep baby entertained while you pack, etc.)
Optional: Portable crib (if the hotel does not provide crib or if you feel more comfortable with your own), sheet, mattress pad.
Neck support and SIDs preventer
Baby or child's book
Thermos for hot water
Small bucket with liquid soap and cleaner brush (necessary for cleaning bottles which you need to use multiple times and for avoiding bacteria)
Baby-proofing items: electrical socket plugs, doorknob covers, sharp edge covers. Consider bringing a safety gate to keep baby or toddler from off-limit areas.
Screwdriver (for tightening loose screws in hotel crib, should the need arise)
A child-care book dealing with children's health, symptoms, and remedies

For the nursing Mom:

Nursing pillow
Nursing blanket
Nursing pads
Nursing bras
Breast pump
Extra bottles for storing milk if the hotel has a refrigerator
Ice bags for the milk (for day trips or going to restaurant)
Nursing shirts