“will be exploring” or “will have explored”

Both of your alternatives are acceptable, depending what you want to say.
Either you will be exploring, or you will have finished exploring, at the time the recipient reads it.

Probably, by the time you read this, I will have unpacked and explored the town before it got too dark.

"I will have..." so the 'will have is 'understood' in 'will have explored'.

Probably, by the time you read this, I will have unpacked and be exploring the town before it gets too dark.

'be exploring' : change of verb from 'will have explored' so we need to use 'BE exploring'.

It depends upon the phrase you want to say if you wanted to tell them that you will explored the town before it's too late then it should be "will be explored" or if you wanted to tell them that you will be exploring then that should be used.

Don't try to advise anyone else on English until you sort your own out. It's pretty **** btw.