Kawaguchiko Lake: Best way to reach

I'd like to spend a day at Kawaguchiko Lake:
1. What is the best anche cheaper way to reach it? It seems bus, can you confirm?
2. Should I book in advance?
3. Can I use Suica Card?
4. About Hotel Fujien: I'm interested only in baths, can I go freely without reservation?

Depends where you're coming from. If you're traveling by JR Pass you can go to Otsuki and catch a Fujikyu Railway train from there for JPY 1140. The station entrance is right next to the JR station. I considered the bus but for me the train was easier. It's a local train, with seats along the walls, and plenty of room to hold on to your luggage in front of you, no reservation req'd. Do your homework: check out Hyperdia, check out bus schedules, and decide what works for you.

If it is solely for a day trip. Then train is the best option as if allows more flexibility for your return. As you don't have to worry about what time you need to depart Kawaguchiko. The trains are every 20-30 minutes. So you can return a 6pm, or stay and have dinner somewhere, and return at 9pm for example. Last train is around 10pm from memory. But you can check on hyperdia to confirm.

The last train connection from Kawaguchiko to Tokyo leaves at 21:19 (if you take the 22:09 train out of Kawaguchiko, you can't make it to Tokyo before trains stop). It requires two transfers, and you arrive in Tokyo around midnight.

The last bus leaves at 20:15, and makes you arrive in Shinjuku at 22:00 without transfers.

Sorry, I forgot to write that is a day trip from Tokyo. So I was evaluating these buses http://highway-buses.jp/fuji/ that seems faster compared to trains. But I guess I couldn't use Suica here and reservation is required.

May this year I used Narita Express from Shinjuku directly to Kawaguchiko. It was running on selected dates only.

You may want to check if some NEX still travel to Kawaguchiko.

N'EX timetables are here: https://www.eki-net.com/pc/jreast-shinkansen-reservation/English/wb/co...

Yes, there is something but depart too late. Anyway I was considering these buses http://highway-buses.jp/fuji/ that takes about 1h 45 min compared to 2h 30 min and more of trains.
And also the price is very different: 4930 yen (train one way) vs 1750 yen.