Non profits

I am planning to volunteer for non profits to make myself occupied. I volunteered before, for more than 2 years., though it wasn’t NP. I am definitely seeking full time, but how valuable will this NP work will be for future career? Will I qualify for mid levels or still be viewed as entry level? Some company that I went for interviews asked about salary history and expectation. Since I have no salary history and only volunteer work, they view them as “not so serious” jobs.

Also I would like to know about non profits design groups. Some that I know are Project H, Whirlwind Wheelchair. Could someone add more to this list?