Rabbi Linzer, in your snow teshuva I don’t think you clarify if there is any problem with adults sledding on Shabbat. Can you clarify in which situations adults would be permitted to sled (with their children? on their own?). Also, is there any additional concern on a designated sledding hill, where the inevitable packing of the snow under the sled is actually a desired improvement as it makes the hill better for sledding?
I don’t see any difference between children and adults. The key is that it doesn’t take you out of the consciousness that it is Shabbat (adults can help children in this capacity). I think adults on their own should think twice, because although uvdah dechol is not precisely defined, it seems to me that as adults we identify Shabbat with rest (and restful entertainment), but not with exerting activity, even if it is fun. I suppose it is the same question as, say, playing tennis on Shabbat.
As to packing the snow and making it better, I am assuming that this is an אינו מתכוין דניחא ליה. The first question to ask is whether it is פסיק רישא or not. I’m no an expert in this, but I would guess that each individual act is not a pesik reisha, as one might be sledding down already tamped down snow, or might be snowing down snow that is not close to being tamped down. I would imagine that it is hard to determine in any given sledding if this is going to have some positive effect in tamping down the snow, which would make it not a pesik reisha in any individual act. However, some might argue that since it will be achieved during the whole process, that makes it pesik reisha, which is how I always understood those who forbid brushing hair for this reason (the original case that brushing hair is forbidden on Shabbat is from a women doing preparation for mikveh, where there is a specific desire to pull out knotted hairs).
So, it is likely eino mitkavein. One could go further and argue that it is mitaseik, if the sledder is completely oblivious to this outcome.
Now, the real question is whether it is assur or not. It is not like tamping down dirt, which could be either boneh (hence the issur of משוה גומות) or choresh. Here, it is all temporary and eino mitkayeim. At most, we are dealing with an issur derabanan, and even that is questionable, since the melakhah might fundamentally be limited to קרקע. (Although, if the dirt is hard and long lasting, it might be assur derabanan. See the Har Tzvi on shoveling your walk).
The bottom line is that we are dealing with something that might be assur derabanan, is most likely an eino mitkavein which is not a pesik reisha. In other words, unless you have kavanah to do this, it is fine.