Our welcome bags are a work in progress, since we have over a year. This is what I am thinking, so far.
Welcome Folder with
Park maps
Last newsletter (I have a lot of fun doing these)
Event/Transportation Schedule/Park Info (going to look like Times Guide)
Hidden Mickey Guide (done, just have to print)
Post card - (maybe personalized to send back to us)
Door Hanger (design done...construction another issue)
Hand Sanitizer (great sale at BBW - already done)
Pins - We are doing alot with Disney pins. Our save the dates are incorporating custom lanyards, so we have decided to hand out pins at each of our events. The pins are based on which character has been assigned to each event (our enclosures and invites will reflect these). In the welcome bags, will be each guests characters pin and what we are calling their year of significance pin.
We are assigning each of our guests a character based and giving them their character pin to wear as well. We have had a lot of fun with this project. Some are sweet and some are not so sweet.

We have several assigned villians depending on the person.
The Year of Significance pins is a way for us to acknowledge each of our guests. For married couples the year is the year they got married. For our single guests, it is the year we became friends and for the kids attending, it is the year they were born. I then went and found something for those years that happened in Disney history (some were very tough) and got them a pin for that event.
We will also probably have the Disney "I'm Celebrating" pins in the bags as well.
We are planning on doing custom t-shirts. We wanted to do this for our first wedding and couldn't afford it, so we are doing it now.
A small bear necessities pack with gum (since Disney doesn't sell it)
A deck of personalized playing cards that I just bought on the knot. They were cheap and I think most of our guests will love them.
There may be more, but right now, that is what we have in mind.