Traveling with a Baby

Help! I'm feeling so overwhelmed! 

I'm traveling to a different city for a training. I'm bringing with my Sam, and my little sister. We will be staying in this city for four night and five days. I've been to this city before, and know it for the most part, what I'm nervous about is under packing. I am a notorious under packer. Matt is a notorious over packer. 

It's hard to find a the happy medium whenever you travel. You either end up telling yourself "I wish I would have packed that" or "I guess I didn't need to pack that".

I really don't care about under packing for myself, however for my sister's sake, I'm worried about under packing for Sam. (She will be taking care of him during the day while I have class.)

So far this is my list:

• Pajamas(6)
• Outfits(6)
• Socks(6)
• Robeez
• Onesies(8)
• Hat
• Swim Trunks(1)
• Baby Towels/ Washcloths(2)
• Baby Blankets(3)
• Diapers(60)
• Wipes
• Bibs(6)
• Baby bowls/spoons(3)
• Baby Food (15-assorted fruits/veggies)
• Baby Cereal
• Expressed Breast Milk
• Burp Cloths(3)
• Baby Bottles (2)
• Baby Bottle Brush
• Toys
• Pack N Play
• Play Yard Sheet
• Stroller
• Bumbo
• Baby Was/Lotion/Desitin
• Nail Clippers
• Ear Swabs
• Infant Motrin/Tylenol
• Baby Orajel
• Sunblock
• Swim Diapers

This seems to be so much! I want to be prepared, but this seems sort of over kill. On the other hand, I really do not want to have to buy something while down there, that I know I already have at home.  I know that everything I put on here, I will most likely use, but still. It's a little much? Do you have a must have while traveling with baby?

 If anyone has any tips with traveling with a baby, or advice, please let me know!