Is there any tool you can use to send custom Activities?
For example, If I wanted to see how lemmy handles an announcement of an announcement, is there a tool I could use to write a custom activity and send it to the target server?
curl
You will need a server that can serve you actor URL and public key. Even a static webpage will do. If you have that, then a simple curl could be sufficient.
I had tried stuff similar to this (hosting static json on my neocities site), but it failed. It probably was because of the keys. I'll try it again with them.
Another thing to consider is that most servers only accept HTTP requests that are properly signed.
Maybe you can take a look at Fedify if you know your way around javascript development? If python is your thing, maybe my own Django ActivityPub code?