How to get to Lake Brienz and Swiss open air museum from Grindelwald
 

I'll be staying near Grindelwald station. I'm thinking of visiting the Swiss Open Air Museum and Lake Brienz on same day. Which station should I stop?? What's the best way to go to these places??

Can it be done on same day??

Use the wonderful planning options of the Swiss rail system -- which also covers ferries and buses and just about everything else that moves in Switzerland and is public transportation!
http://www.sbb.ch/en/home.html

You can take a boat ride along Lake Brienz and visit the Ballenberg Open Air Museum on the same day -- get off in Brienz and take a bus from just outside the ferry station.

The Ballenberg Museum has two major entrances, so either get off at the first, walk through the park to the other, and catch the bus back from there -- or vice versa.
http://www.ballenberg.ch/en/visiting...erg-site-plan/

Can it all be done in one afternoon ?? The lake cruise and museum. Howe to get back from Ballenberg museum to Lucerne??

Can it be done? Depends on what you want to see and experience.
How to get from Ballenberg to Lucerne? Check sbb.

Honestly, I think its time for you to do a bit of your own homework. JMO.

"Can it all be done in one afternoon ?? The lake cruise and museum. Howe to get back from Ballenberg museum to Lucerne??"

Lucerne or Grindelwald? What about your luggage?

The boat is just over an hour one way from Interlaken to Brienz, but you really want a full day at Ballenberg - it's quite spread out and there's a lot to see. You can get the Lucerne train direct from Brienz station (2 hours), but please note there's also a bus journey between Brienz and Ballenberg although it's not all that far.