This is a two part question. First, how exactly did you introduce your first (or any older) child to LO? I have searched some ways and the most popular seems to be in the hospital when baby is out of the room, have DS cuddle in the bed and then have baby brought in. And there is usually a present from baby to older child involved. DS will be 3.5 when DD is born.
Second question is for after LO is home. What were some of your most successful ways to occupy older sibling(s) while taking care of baby?
Re: STM+, what to do with other child(ren)?
ETA: This time my parents are coming to get the boys when we have our c-section or if they start coming sooner. I'm hoping they will hang out at our house until the twins actually arrive so that the boys can visit early on, but if not that will be okay too. They'll just come in when they can to meet their new brother and sister. DH is going to be on big kid duty when we get home and they will probably luck into going to work with him a few times here and there which they love because DH's clients always dote on them and give them way too much candy when they visit.
As far as occupying your other child, I let him hang out with me while I took care of the baby, nursed, etc. Sometimes he wanted to do his own things and play with his toys. Sometimes he wanted to cuddle up on me. It just depends on their mood at the time.
Just feel it out and trust your gut. There will be moments that are calm and moments that are totally chaotic. It's normal.
As far as activities, I think it's different for every kid, but what keeps my son occupied the longest lately is play-do and puzzles. Sometimes watercolor paints, Legos, or eating a snack.
ETA: we introduced ds1 and ds2 the same way. Ds1 was 21 months.
As far as entertainment goes, I'm going to be making her a "big sister kit" with a sticker book, snack, maybe a dvd, and a disposable camera so she can take her own pictures of this special time.
I'm starting to really worry about entertaining DD while having a newborn. She is such a busy little girl and has to be doing something 24/7 but thankfully she does also play very well by herself. Hopefully she will adjust well.