I also really enjoy just following the instructions and building a set. Very relaxing. But the sets started to pile up so I no longer buy any. I wish there was a good way to build a set, enjoy it for a week or so then return it. There is one LEGO rental company, but I never took a shot on them due to some bad reviews.
LEGO offers instructions for all their sets, so having parts opens new possibilities: https://www.lego.com/en-us/service/buildinginstructions
Of course, there are some special parts (like robot joints), one can buy them online or improvise with existing parts.
Actually, you could keep all the pieces mixed up in a giant bin. That's what we do. As my daughter said, "the fun is in finding just the piece you need"
