I play with a sort of middle ground between having an actual plan and just doing whatever. I generally have a sort of to-do list with various goals that lead into larger goals, but will constantly switch between which parts of those I feel like doing.

For example if my main goal is to beat Bonemass in this scenario, it will have various sub-goals like gather mead ingredients for poison resist, health, and stam potions (functionally, wander around randomly in various T1-T3 biomes grabbing a bunch of ingredients whether they are for these specific meads/potions or not), acquire good fight foods (functionally, serpent hunting for the fun of it and the cooked serpent and serpent stew and farming, onions if I'm lucky), make a temporary forward base reasonably close in case the worst happens (includes scouting the boss area and constructing workbenches to minimize spawns joining the fight beyond what I already have to deal with), prepare weapons and armor (mostly a bunch of crypt farming for iron, maybe a visit to the merchant so I can go make the iron sledge as well).

I never specifically focus on one at a time, just sort of shift between these goals as I feel like it. If I feel like building that day, I'll work on that forward base goal. If I feel like fighting that day I'll probably go raid some crypts. If I feel like just putzing around then I'll probably work on ingredient gathering and farming. The serpent hunting bit tends to be a personal past time and just sort of happens when RNG allows as I sail between various places or search for leviathans because I wanted a harpoon and knife. If I really don't feel like working towards anything in particular, it's time to explore some more. It all ultimately works towards my main goal at the time, but still lets me just sort of do whatever and progress at my own pace.