The cave is accessed via the green line (about a 25 minute drive).
Entrance is 350JPY per adult though the 2-day bus ticket gives a 50JPY discount (this is true for all the caves).
Before you get to the cave entrance you take a short stroll through the edge of Aokigahara forest, which is lovely!
The cave itself takes around 20 minutes to complete if you take the hard route through it. It involves a bit of crawling as the ceiling gets incredibly low (you can walk around that part and use an easier route, as the cave is circular) and I highly recommend taking it since it's more fun. Around the cave you witness various kinds of formations created by the lava flow that are amazing! Also, despite many reviews stating that "no bats are present at the cave", my friend and I saw two of those tiny cute things sleeping at the deepest end of the cave. You just have to take a glimpse into the barred crevices that are blocked from entry.
With or without bats, the cave is worth a visit just to see the formations and crawl around! It was the one we enjoyed the most out of the three (the others being the wind cave and the ice cave).

The cave is a good place to start a hike through Aokigahara forest. The hike was the highlight of our day and a must for anyone visiting the five lakes!

There are no bats in this cave but it's quite interesting. You'll need to wear a helmet to enter, and the ceiling is so low in some parts that you almost have to crawl!

It was nice to get out of the center of Kawaguchiko to take a walk through the bat cave. Although no bats seem to be inhabiting the place at the moment, it was cool and educational to walk through. The cave isn't huge and you'll be done in about 30 minutes, but it doesn't cost much and the bus ride out was scenic (and a bit long).

The gift shop was nice, the staff were helpful (and spoke enough English), and the surrounding area is lovely.

Also, there are a lot of trails in the area you can walk on, and as this is just at the edge of Japan's (in)famous 'Suicide Forest', it might be an interesting story to tell friends and family later.