Assuming that Biblo really is using the word for "tool" and not "weapon", I'm not expecting any of Joyboy or Nika's old weapons to be in there. It's probably not as simple as a sword or a spear or a shield.
...Then again, you COULD say that the Ancient Weapons were "tools" to be used for some other purpose, back in the day. So, there could be some sort of overlap, here. Something you could use as a "tool" or as a "weapon", depending on how you choose to use it.
If it's from Biblo's Grampa's time, that means it's OLD. Like, Void Century or First World old. Biblo's already older than any Giant can remember, and the Giants are already pretty long-lived. The old worlds had more high tech stuff, back in the day. So, whatever this "tool" is, it's probably something surprisingly high tech.
This could be something from the same age as the 3000 year old lab. So...maybe something a "scientist" would use?
I could see it being some sort of force field or EMP. Something that would force anyone with a "Steel Heart" out of Elbaph. At least temporarily. A big button that you could press to end this arc faster than expected. A "get out of this battle free card" for the Strawhats.
Maybe it's some sort of "universal translator"? That'd make some sense to keep in a "library". Something that could translate the ancient script on the letter and/or translate what Biblo's trying to tell everyone so he can talk to everyone at once.
Maybe it's something as simple as a "iPad"? Some sort of computer. Maybe the "letter" is actually a video message on this tablet?
Maybe it's a "phone" or a..."high tech" Den Den Mushi? Something that would let the Strawhats "call in" another robot like Emet. Or a army of robots?
This treasure chest is pretty big, though. So...maybe it has some sort vehicle inside it? Like a jet or a maser cannon truck like you'd see in Godzilla movies.
Maybe this is basically the Ancient Giant equivalent of a "toybox", with a bunch of "toys" inside?
I have speculated before that Imu could have been the one who drew the Chapter 1138 Mural on Elbaph. That...seems less likely now. But, maybe there's still some sort of connection Imu has with Elbaph? Maybe this box belongs to Imu? And maybe the idea is that it's full of Imu's old treasures that would make him remember what he used to be like in the past? A "time capsule" full of things that Imu used to treasure that would remind him of his old self? Maybe dredging up enough old emotions that he'd stop attacking Elbaph? It's unlikely...but I'll throw it out there.
https://worstgen.alwaysdata.net/forum/threads/is-imu-the-one-who-drew-the-mural.80832/
It would be kind of interesting if the letter WAS the "tool", at the same time. A letter...made of the same material as a Vivre Card. A letter that would lead you to someone...waiting at Laugh Tale. That's VERY doubtful, especially since there's no reason for the box to be that big for one letter. But, it would make for a interesting twist.
Since this supposedly belonged to Biblo's Grampa...maybe it's some sort of pet bird toy? I'm not sure what. Some sort of hi-tech ball? I mean...some bird toys DO happen to look a lot like the World Government Logo. That could be...something?
https://www.amazon.com/dp/B0BRGJRRXD?ref=clp_hp_h_pc
Although...this entire setup DOES feel suspiciously familiar. A treasure chest being kept secret in a dark room, that only a animal knows about? This seems VERY similar to how Loki found the Nidhogg Fruit that Ragnir was guarding. So, maybe this "tool" is actually another Devil Fruit? It'd be...strange, to call a Devil Fruit a "tool". But, we do know that Devil Fruits are basically ancient technology. So, it's not totally inaccurate. And we could use another powerful Devil Fruit to fight Imu. A Devil Fruit would be worthless to Robin or Chopper or even Biblo himself, since they already have one. But...Saul doesn't have a Devil Fruit. Could this be setting up Saul to get a powerful new Devil Fruit? Maybe even a "God Fruit"? It'd be kind of fun to see Saul get a chance to get a few licks in, since Imu kind of ruined his life.
It's almost impossible to say, right now. We have so little to go on.