We felt the same about having an adoption shower- it was an important rite of passage for us and made us feel special. Ours didn't end up happening because LO had to be moved in early, but it was all planned and we had it a year later to celebrate the A.O anyway! How lovely!
We set up an Amazon wishlist so that people could send stuff directly to us based on what we knew they would need and the kids ages. Our kids came with loads of stuff from FC (although some APs I have spoken to said the opposite). It had the added advantage that we got to be specific and people could use the link and get us stuff from elsewhere if they didn't want to use amazon but knew it anyway.
Everything depends on age for gifts.
We really valued the National Trust membership we got given, and we have a zoo membership as well, which has been amazing. We also needed lots of practical bits like anti escape seatbelt clips, toddler locks for doors, nightlights, disco lights and reusable bed wetting mattress protectors in funky colours that we were able to get nicer versions of from other people than we could have afforded otherwise. (Happy to list more stuff for gift ideas if you want, im aware thats not what you asked for! Approximate ages would be helpful though) Deliveroo vouchers helped us through some rough introduction nights as well! We also really liked books, and we obviously use them every day. You could have a "bring a book" with messages written in the front in instead of a card maybe?
In terms of tips, it does depend on what the person is like- we had a 10 question quiz that included some typical stuff like guess the weight he was when they were born (which was fun because we already knew the answer) and what flavour icecream they liked best so my grandma could add them into the back of her bible where she writes all the great-grandchildrens birthday and birth weights. We also had they typical "write a message in the nappy" because ours were still in nappies, but that could be write a midnight message.
My sister made sure that people bought appropriate adoption cards by sending round links online and then taking my grandparents to buy cards in person because it felt important to us not to just have new baby ones and my family can be very insensitive with stuff like that.
I don't know whether they have any family members/friends who might be a bit insensitive, bit it might be worth offering to speak to anyone who might say something and in a friendly way lay out what is or isn't acceptable. (You would be amazed what some people feel free to comment on, and we felt so vulnerable at that point). It's horrible to think that a milestone like that will be tarnished with someone asking when the kids will really be theirs, or why they have been adopted etc. Someone else doing that emotional labour can be really valuable if you have that sort of relationship.
For decorations we had planned to have photos of the boys and a brief bio for them - something like "Jim likes tractors, fire engines and saying the word poo" so that people knew a bit about them, but it may not feel appropriate for them.
I'm so happy for you and your friend- we were disappointed ours didn't happen when we wanted, but the planning that went into it and all the stuff around it showed us how much people cared, and the one a year in was just as special to us and created so many memories that are really important to us as parents.
