first time to cr should I book accomadation in advance bring American dollars are there buses from san jose in evening if have late flight getting in any yoga classes
If you find a hotel you really like you might as well book it. That being said, we didn't book ahead in that area but we went in March.
If you are Canadian like us I wouldn't bring USD. You can use your debit card to get colones from the ATMs.
There are no buses in the evening. The last bus goes at 4 pm. Best to stay in Alajuela for a night (Alajuela is the town closest to the airport, nice town).
I am sure you can find yoga classes and hotels that offer yoga.
Cahuita has an amazing National Park. Maybe after Cahuita you could check out nearby Puerto Viejo and the beaches south of PV.
Another fun but more expensive way to go from San Jose to Cahuita or Cahuita back to San Jose is a rafting trip on the Rio Pacuare (the tour company picks you up at one end, takes you rafting and drops you off at the other end).

How long do you have for this trip?
Concur, if arriving late stay in Alajuela for the night, cool place and easy to walk around to places.
Oct. should be easy to walk up and get places, what's your budget and lodging style?
Tortuguero is another outstanding place.
Yoga is everywhere, no problem.
Fun cheap day, rent a bike in PV and ride down to Manzanillo, have lunch-several beers and ride back. Good way to check out the variety of places along that stretch.
The Pacuare is outstanding and worth it, IMO.

I am not sure of the exact dates but for a week in October there is Carnival in Limon (although I read it was cancelled last year) so town may be filed up with persons visiting Carnival but staying out of town from Limon (which is always a good idea).

I was thinking in the 40 to 50 dollar a night and have a budget of 100 a day I just need a clean simple room and good laid back hosts
Usually Oct is rather slow, though per above, there is Carnival week there which is very busy. You may want to consider Pt Viejo area, and beaches south, as its more choices for lodging and plenty of Yoga.
You budget is more than enough too...
Cahuita I would go 1-2 nights...
Any interest in Tortuguero Canals and Rainforest on the way to caribe for 2 nights, gorgeous area with a lot of wildlife/nature.
October is the rainiest month of the year in CR, so be prepared, and expect delays. Be aware of Dengue Fever Outbreaks, and Zika issues.

Your budget is excellent, you'll find killer lodging for 40-50, in the jungle or on the beach, depending what you like.
How long is this trip?
The longer you stay at a place the better price you'll get.

is it rude to barter over accomadations if mostly empty im a slow traveler and will hit probably 3 places in two weeks whats your top 2 things to do im good at chilling
Hi, How old are you? Cahuita is a good base for someone older than their thirties or so. Puerto Viejo is for the younger crowd. Both are as short bus ride from each other. Yes, bring some US dollars but mostly use the ATMs. There is one in Cahuita. If you are laid back there is plenty do do in two weeks. Lots of beaches, restaurants, bars, the parks, ect. In October the hotels will be pretty much empty so yes I would try to bargain if you think the price is too high. I wouldn't pay more than $30US/night for a place with a private bath.

is it rude to barter over accomadations if mostly empty
Of course not, just don't be a dick about it. Be nice, like what's your best deal, offer cash and to stay for multiple nights.
Best way is to get a place your first night, dump your gear, and check out all the places, ask to see the room. You'll quickly get a good idea on the price for a nice room and location. Don't hesitate to hit the higher end places as well.
The owners are used to dealing with bargain hunters. Some might offer a free bike during the stay, 1/2 off on yoga, never know.
im a slow traveler and will hit probably 3 places in two weeks whats your top 2 things to do im good at chilling
What do you like to do, besides chilling. Hiking through Cahuita NP is a lot of fun, easy and cheap. Bike ride to Manzanillo from PV is a good chill day. Get a kayak is Tortuguero and chase gators.