I hate the risks of trampolines. So you lost me on that one. Op, what I found is my kids filled-in with experiences on their own. And had motivation to do so. They went skiing in their 20's. It took them a couple times before they were good. Sure, there were times the thought crossed my mind that I wished we had exposed them to skiing. Same with travel to the West Coast. We went to Europe but not the West Coast. They each went in their 20's. They also traveled on their own to destination that, I never would have anticipated they would want to experience. They can buy their own trampoline. Or their kid a trampoline. If if turns out that's what makes them happy.

My kids are older teens and I have to say that I have no regrets. We did what we could with the money we had. We took advantage of many of the free activities (museums, concerts, etc.) and supported the in the sports they wanted to play. We couldn't afford private coaches but they did camps at the rec center and workshops offered by the leagues. DH and I also learned as much as we could and were always willing to work with them in the back yard. We got used equipment from Craigslist/Facebook Marketplace and set it all up in the backyard/basement.

As far as adventures, I'm assuming you're talking vacations to other locales. We've done none of those. Not in the budget. Our trips have been to visit family and at those visits, we've done the touristy/outdoorsy stuff. We still do day trips with the kids (older teens/young adult).

Do I wish we had the money to buy great gadgets, take amazing trips and have personal coaches? Sure, but we have a healthy home life, supportive relationships and modelled good financial practices. My kids have had a really good childhood.

More outdoor time
More free, unscheduled time

I wish I had managed to take my kids on more trips and outdoor adventures when they were younger. We did do a lot, but that’s what I love, and what I was really eager to share with my children, but now my young teen both is less enthusiastic and has commitments to activities that make it hard to go away as much as I would like (and soon enough he’ll have summer jobs too). We did the best we could though given we had more limited funds and vacation days when they were younger,

We always followed the kids lead, as far as whatever they were interested in. Tried a lot of different things, some stuck, some didn't. I did say no to a week long ice hockey camp, because I would have had to buy all the gear. For one week!
We also said no football (concussions) and no trampoline. But: skiing, tennis, baseball, soccer, basketball, volleyball, horseback riding, dance, voice, instrument, theatre, sure. None of them were ever interested interested golf and that would have been good to learn, but they were never interested. And given that they are now both in their 20s, I am sure I am forgetting somethings they tried at one point

OP there are always regrets, especially in hindsight. Don't beat yourself up.

I wish I could have figured out what activity might have led to close friend relationships for my middle child.

As far as trampolines, not to pile on to OP but rather to respond to the naysayers, our trampoline is one of the reasons my kids are really close. Every evening after dinner they would hang together on the trampoline. If the weather was bad, they’d kick us upstairs so they could hang in the kitchen on their hoverboards. No regrets for taking the risk.

Both the American Academy of Pediatrics and The American Academy of Orthopedic Surgeons advise against trampolines. Nets only help with injuries from falling off.

We never forced out kids to play sports but we always encouraged and supported them when they did. I do regret not exposing them to more diversity and education opportunities like museums but in general I think we did a good job. They were never demanding about “stuff” so when they did want something special we usually got it. But, I too would have been concerned about a trampoline but thankfully now that have them with big netted fencing.

Don't feel bad about the trampoline, OP. They are extremely dangerous and a liability to have, and they are really unsightly too. You had very good reasons not to buy one. Your kids can use them at friends' houses.

I kind of regret not letting my son do Boy Scouts, but only kind of. I know this won't be a popular opinion but my husband and I dislike the BSA organization (the hypocrisy of refusing to allow gay kids/leaders because it wasn't "morally straight," but meanwhile kids were being molested for decades by "morally straight" adults and it was covered up). I know they have cleaned up their act in recent years but we still don't trust them for safety or want to support them. They also expected a huge time commitment from families, which we didn't want to invest. But I know my son would have loved it, he is outdoorsy and athletic, and he's definitely given me guilt trips about it, especially now that he's older and sees his scout friends going on these cool trips. Maybe I should have just held my nose and sucked it up.

Your attention span is crap from social media

Skiing, swimming and golfing more

They can do those things but not well

More horseback riding. Group classes aren't more expensive than their individual music lessons.

No regrets really. DC skis ice skates rides horses swims like a champ bc we introduced all these sports to them before the age of 7. They also played softball basketball and soccer - for long enough so they do a pickup game anytime as an adult. We put in an inground pool that in one summer made up for YEARS of ineffective weekly YMCA swim lessons. We didn’t have a trampoline. They went to a friends house for that. And that friend came to our house to swim. I guess….They aren’t great at riding a bike, I guess I could have pushed that more? But they picked up skateboarding on their own with friends. At the time that we were doing all this we didn’t have as much income as we do now so it felt like we were hemorrhaging money but very glad they have these experiences and skills now.

I am looking at a bare spot in our yard where I should have put a trampoline for my kids. They begged for one, but I was always afraid that someone would get hurt. I also selfishly thought that it would ruin my garden. My neighbors have them and their kids are always out there bouncing around, laughing away. It seems almost therapeutic for them after school.
I also wish that I had put my son in organized sports earlier. He was not a natural, and earlier involvement in everything from skiing, to soccer would have been helpful.
I can't reel back time, but I can share this with those who have younger kids.