For scaling, I recommend having the items get more powerful as you find more of them. That way, the artifacts they find at levels 3-4 are still relevant at level 12. Maybe if the players pass an investigation/arcana check, they could use their existing artifacts to help determine when they're in the proximity of a new one.

So if you have an orb that is full of swirling clouds, it could let them cast Stinking Cloud at DC 12 at level 3 once per day. Then as more items are discovered, the DC would increase, and eventually it would cast Cloudkill with a DC of 17.

Other artifact ideas:

• a long, thin strip of leather (characters with a background in smithing or proficient in leatherwork recognize it as a handle grip). When wrapped around the handle of an ordinary weapon, it becomes a +1 magical weapon (upgrading to +2 and +3 with more artifacts).

• a gemstone that seems to absorb light. It allows the bearer to cast Counterspell once per day (with more artifacts, the Counterspell is cast at a higher level, and the gem gains additional powers, like being able to create an anti-magic sphere once per day).

• a figurine of a hammer. Once per day, it can be used to instantly repair a broken Tiny item (with more artifacts, the size of the item can be Small, then Medium, then Large, etc)

• a burnished shield that looks like it's never been used. It grants an additional +2 to AC (+4 total), and once per day, it can absorb all damage for a turn (with more artifacts, the ability protects others around you within an increasing radius)

• a charred staff. The bearer can cast Scorching Ray once per day (with more artifacts, it casts Fireball, Wall of Fire, Firestorm, etc once per day as well)