There are several main items that need to be in your diaper bag at all times! Review the list below for what I have found to be the most useful:
1 full footie pajama/outfit
2 Onsies
1 pair of pants
2 pairs of socks
6 Diapers (and a stash of 3 more somewhere else in the bag)
50+ wipes
2 Burp Cloths
1 Large Receiving Blanket
1 Small Receiving Blanket
1 Backup Binkie/Lovey
1 Bulb Syringe
1 Container of Gripe Water
1 Medicine Dropper
1 Age Appropriate Tylenol
Assorted Toys
This is all minimum. Obviously many of us have children with different needs. For example when my son was younger and had GERD I needed a lot more outfits and burp cloths.
Practical Point: consider keeping a separate traveling container for toys as they can be awkward and take up too much needed space for necessary items.