First and foremost, do not take control away from your friend with any of this, however well meaning. Dying is the ultimate loss of control over everything, so being able to still have control of small things becomes incredibly important. Even if it's just what you want for lunch, for example.
If they want support or ideas, I'd add, on the emotional side...
Memory boxes, if possible created together, but also if the children are young (or anyway) leave a letter explaining why those objects were important and special and the memories attached.
Include items about her life before the children and the person she was so they can feel a connection and a way of knowing her. Leave behind stories.
A photo album of her favourite photos of them, her, them and her, them all together, etc. Include notes if possible explaining why she picked them.
A letter can be comforting. If she wants to write a letter, it's worth bearing in mind things can happen rapidly so hard as it is it may be better to do sooner and be able to hand write it or risk running out of time to write it at all or having to dictate it to someone.
Cards or letters for significant milestones...
If there's a particular poem or something that holds a comforting message that could be passed on once they get older. You can get little books of a single illustrated poem for instance.
I'd second doing one trip/activity of choice for each child.
It's exhausting enough emotionally to handle this, let alone what's going on physically... The kind of frenzied must-do-everything activities that make the media aren't really the common story.
There are some things we can't fix, but we can be there. Just be wary of trying to "fix" this by launching off into lots of practical things or trying to take over and do things without being asked... Please, please prioritise ensuring they keep control of everything.
There will be a lot of small things they will be losing control of right now, most of which you'll probably never see or know, so although I know you mean well please just be conscious that stepping in and taking away the remaining little things they're trying to hold onto could have a devastating impact. Offer, support, but don't take over or make decisions.
And take care of yourself.