I’m planning a trip for a mixed group, and was curious if you guys would mind sharing experiences you arranged/chose at WDW that were super memorable for you. What are some not miss things to convert everyone into the Disney addict that I am? 
It will be four adults in their 30s-early 50s and three kids 6 and under. We will be staying at the Poly.
We loved the fireworks dessert party but idk if that’s even still happening.
The dessert parties are still happening but they have increased in price (scroll down to bottom of page for the other row parties)
https://disneyworld.disney.go.com/dining/magic-kingdom/tomorrowland-terrace-fireworks-dessert-party-plaza-view/
During our last visit we really enjoyed doing a self-guided Skyliner “sip and sweets crawl” during non-peak hours on a non-park day.
We had a beverage or dessert at each Skyliner resort which was a really fun way to see different hotels and get our bearings for our first trip to WDW. We started with drinks at Beach Club then took the International Gateway station to get pastries at Riviera and more snacks and drinks at AoA and Pop Century. The ride back to IG just happened to be perfectly timed with the sunset and was really beautiful. The experience was enjoyed by kids and adults alike.
I know you’re staying in the MK area but if you have an Epcot day planned, the Skyliner is a nice break.
A private fireworks cruise is 100% worth the money.
The Wild Africa Trek is widely regarded around these parts.
I no longer recommend the fireworks dessert parties. But I do endorse @LTinNC82’s suggestion of a fireworks cruise, which would actually be cheaper for your group.
What are the big interests of your group? My DH was excited for Galaxy’s Edge so I planned an evening specifically of exploring, riding the two rides, building a light saber, and going to Oga’s. We ended the evening with a light saber fight in front of a totally empty Millenium Falcon. It was perfect. But only if you are into star wars!
The cruise sounds fabulous but alas I don’t think the kids will appreciate it, since it is so far past their bedtimes 
Will keep it in mind though!
Kind of all over the place. One into princesses, a couple Star Wars fans, one more into pampering and day spas, etc. Kinda makes it hard to find one thing that would impress everyone.
If the Tea ever opens at Grand Floridian you could do an afternoon at the Grand Floridean with some at the Tea, some at the spa and others get cocktails at Enchanted Rose.
My ideas are maybe on the lower end of the fancy spectrum but fwiw…
Honestly… Chef Mickeys breakfast! There is something so magical about how excited my kids get to see the characters. (Ok let’s be honest, I get really happy too!) Then either walk to MK or monorail to Epcot, such a smooth and easy way to arrive to either park… anyway you can show your guests how you are breezing through security is sure to win them over.
Also when I took my mom in 2018, was both of our first trips as my parents never took us as kids, she really liked Kilimanjaro Safari and the Nemo show. The Safari is just so extra for a theme park, depending on the frame of reference of your party, it truly feels like you are somewhere else. She was also impressed by the Broadway quality of the Nemo show. For your older guests if the reboot of Nemo is back, that would be something to keep in mind.
We had newbies with us in 2019 - 40 somethings and 3 girls - 5, 8 and 9.
One highlight was Kilimanjaro Safaris. The kids were enthralled. And they’ve been to the zoo several times.
We did Tusker House, Crystal Palace and 1900 Park Fare. Big hits but then these kids love to eat. All those choices! The characters didn’t hurt either.
Another crowd pleaser was Typhoon Lagoon. They stayed near the shallows at the wave pool. Dad took the tall for her age 9 yo on slides while the other two played in the kiddie area.
Towards the end of the week we stayed up late for Illuminations. If 9 pm is doable, the Electric Boat Parade might wow the kids.
Unfortunately, not available for kids under 8.
My youngest is 7. Just how big are the lockers at AK?
Pat overnighting in the mailbox
The Kids Are Alright
Since you are at Poly, one easy and obvious thing is watching fireworks from the beach or marina. That has a pretty magical feeling to it.
Not sure about the size of the lockers but make sure you have a credit card to pay for the locker. We got stuck once because there’s no attendant for the lockers at AK so you can’t charge it to your room.
Never mind, just realized the joke! 
I don’t know if I’d call it “mind-blowing” but the light saber build at Savi’s workshop is really great if anyone in your group is a Star Wars fan!
definitely one of the most memorable experiences for me and my son (6 at the time) 