My answer to this is not at all whimsical. I wish I'd let DS1 struggle and even fail more often when he was little.
(And what I am about to say does not apply to all kids - not to ones who have been abused or neglected, I'd like to make clear - but it does, I think, apply to many or most...)
Our culture is so sold on 'positive parenting', I think, that we focus almost exclusively on making life as pleasant as possible for our DCs, and we forget that they also need to learn to deal with difficulty and unpleasantness. We provide fun and constant entertainment, avoid boredom, support and enable them, encourage and praise, help them achieve, negotiate their friendships and relationships at school, resolve conflicts for them, keep them out of trouble if we can, take them on picnics and holidays, make sure they get plenty of messy play and bouncing and splashing... and try generally to be the very best parents we can be.
But as kids turn into teens and then into adults, they run slap-bang into reality: life is full of difficulties and mummy and daddy can't fix them for you; you need to be able to fix them yourself.
Many teenage problems, it seems to me (and many adult ones, frankly) spring from the fact that our society doesn't give teenagers enough skills to deal with difficulties. Life isn't always rosy, and when they find it isn't perfect, and grown ups can't 'kiss everything better' any more, teenagers often seem to get a bit lost, depressed or angry.
The fact is, as adults, to be happy and functioning and resilient and mentally well, we need to be able to deal with difficulties. We need strategies. We need to know bad feelings pass, and even if we feel absolutely terrible, we will feel better again. We need to know that bad things happen, and that we can sort them out, and that even if we can't, we will survive.
Somehow, my DS - like millions of others, I reckon - reached his teenage years not knowing that he could deal with difficult situations positively. He couldn't deal well with conflict. He associated trying with failure, and hadn't learned that effort brings rewards. He got frustrated easily. He expected me to sort out all his problems.
So, the things I wish he or I had done more of are not the positive things, not the stuff of happy memories. If anything, I wish I'd let him be bored more. I wish I'd let him struggle and strive a bit more. I wish I'd left him to resolve more if his own problems.
When he complained, about boredom or fall-outs with friends or most other problems, I'd tell him "Oh dear, what are you going to do about it?"
Because one of the important things I've learned, as a parent of a teen rather than a small child, is that happy memories are lovely, but that making everything lovely for your child does not make them happy.