I'd say at a minimum, a welcome letter (thank the client for choosing you, recap conversations and note key bits of info), your policies & terms and a copy of your executed service contract. And some business cards. A lot of people throw in marketing materials - so I have read. A very smart woman suggested to me that a welcome package is something that welcomes an engaged client, so at this point, you don't need to keep selling. For sure put business cards in it, because you never know who they might hand them out to on your behalf. And if you have a referral program (i.e. refer a friend and get x benefit from me), that info is good too. If you're going to accept credit card payments manually, include a form.
Like another person commented, think about your business model and build your package around that. I like to always ask myself, does this item make my life easier or my clients? Does it take up my clients time? If it does, I need to make sure it's necessary, or else find a way for me to gather the info without making my client do chores.
Now, all of that to say that I am new too and these are just my ideas and some input based on things that I've seen come up repeatedly in the research I've been doing