Do all of the items need to be directly useful to the players is a roleplay/combat situation? If not, you can make objectively powerful items with interesting effects with short durations, long cooldowns, or effects that scale as the wielder becomes more powerful. You can also give the items drawbacks until their secrets are uncovered.

An umbrella that can cause a light rain, a downpour, or a thunderstorm in a 10x10 mile area with a 1 week cooldown.

An orb/weapon that doubles dice damage for a single combat and causes two levels of exhaustion after the combat ends. Reduce to 1 lvl of exhaustion for characters with proficiency bonuses of +4 or higher.

A shield that summons a spectral guardian. The guardian cannot attack. On each of its turns, the guardian taunts all enemies within 10 feet. Those enemies make a DC 15 wisdom save. On a fail, attacks against targets other than the guardian have disadvantage until the end of the guardian's next turn. Cooldown: 1 day. Scale the DC of the save with quests to uncover more information about the item, or give it synergies with some of the other items which increases the DC when the shield is within 120 feet of the umbrella.

You can also make the items deteriorate with each use until their secrets are fully uncovered.

A skeleton key which unlocks any lock it encounters. After each use, more and more cracks appear. After the third use, roll a d20. On a 1 or 2, the key's magic fails and explodes dealing 8d6 damage in a 20-foot radius. You wouldn't reveal the full consequences of the item, but you could describe the item getting hot and showing visible damage. Giv the players arcana checks to try to figure out what's going on.

The possibilities are endless, and you make the rules. The items don't need to be 100% useful when they're discovered, and they can get more powerful so the players want to keep using them.