Since your mom is still alert, and will help with the party, you might ask her if she has a specific theme in mind...and go from there.
This will dictate the decorations... say, as Court suggested...she liked the 30's ..the bootleg era... of movies and... then...
..you might decorate like a jazz club.. or something we'd see in an old movie. Have cut outs of old movie people.
Invitations could resemle movie tickets... with her name date of party etc.
Other than this, I think you have it all covered... with memory books of pictures...you can bring.. maybe even a dvd...of pictures...depending how good you are at doing this. ( take old movies and have them put on one dvd)
You might have everyone write a short little something to her.. what she means to them and then incorporate it into a book. You can collect that day and assemble later, or if they can get these notes to you... do the book for the day of the party.
At Aunt Libbie's 100th birthday, I had stenciled cake pictures I ran off, with the heading Aunt Libbie's 100th.. and handed them out to all the children to color
Then I collected and put into a book. My daughter in law laminated the cover page..and this was another book
You might incorporate both ideas into one book.. Adult notes and children's colorings..
I'm pretty sure any thing like this would be so much more enjoyed than any material gift anyone could buy.
You still have plenty of time.
Wow, I envy you ... having a mother still alive. My mother passed away in 1976. ( my father in 1974).
My two older children don't remember them at all, and when my youngest, Bobby was born... all his grandparents were in heaven. ( We lost my in laws in '75 and '79).. The seventys were not good years for us.
So, I hope you have a wonderful party.