So 'points of interest' is probably way too broad, so I'll focus on the National Parks.
There are 59 National Parks in the United States and territories, across 27 states (and territories). California has nine of these:
- Channel Islands
- Death Valley
- Joshua Tree
- Kings Canyon
- Lassen Volcanic
Some of those, as indicated in @choster's comment, are debateable whether they're in 'northern California', and it also took me some effort to find lists of numbers. However, there are a few articles around about the least visited parks in the US, most being top 10 least visited, and as such don't have any California ones (pretty much always Alaska). However:
Hidden Gems: 25 Least Crowded U.S. National Parks [50 PICS]
is one list that DOES feature Californian parks, two to be precise - at 15th, the least visited Californian park is Channel Islands, as suspected. However, at 18th on the list with an annual visitor rate of 352,517 (2012) is Redwood National Park. Which is definitely in Northern California.
As a result, that looks like the best option in terms of National Parks, and I'll leave you with the official website for Redwood National Park, which comically leads with 'More than tall trees'. I actually nearly went there a couple of years ago - was between that and Yosemite. Yosemite was spectacular, but now I want to visit Redwood :)