Sharing final plan - need advice

So here's the plan I've got so far.

All hostels/hotels are booked but I'm missing 3 days (10-13/10).

28/9 - Nagoya
29/9 - Takayama
30/9 - Shirakawa Go
1-2/10/2014 - Kanazawa
3-7/10/2014 - Kyoto
8/10/2014 - Osaka
9/10/2014 - Mount Koya
10/10/2014 - ?
11/10/2014 - ?
12/10/2014 - ?
13-17/10 - Tokyo

I'm looking for places I could travel to from mount koya, preferably without losing a day on travelling to destination and back to Tokyo at the end.

I've thought about a couple of options:
1. biwa lake, sounds like a nice location, there are a few places along the lake I could visit and can be a pleasurable quiet break.
2. kiso valley hike - sounds like fun and actually allows me to do some walking (which I like) but couldn't quite figure how to set this up online (things like how do I book someone to pass my bags to the next stop, is it possible to do it the other way around etc.)
3. Travel to Hiroshima and back? It's a good 5 and a half hour ride to Hiroshima (from mount koya) and another 5 and a half hour ride back to Tokyo.

But I'm quite open for suggestions

Since you're already at Mt Koya, visit the Kumano Kodo, Nachi no Taki and perhaps Ise.

http://www.japan-guide.com/e/e4952.html
http://www.japan-guide.com/e/e4955.html

Kumano Kodo seems amazing but I'm unable to figure out how to book the walk.

I found two self tour guide operators, but both are fully booked - and I can't figure out if this is doable as a lone traveller without prior booking