This is a plein air painter’s dream spot. Where to start? Well coming up Highway 18 or the 330 on your way here gives you some incredible views of the floor below. You can pull off in any of the turnouts. You can also pull into the little parks that you'll see along the 18. But you need a "Forest Adventure Pass". You can get it at all the ranger stations and a lot of the businesses up here sell them. On some days you get a view from above the clouds.
The lake itself is pretty hard to access since it’s private but you can sketch it from the coffee shop and various other spots in the Lake Arrowhead Village: http://www.lakearrowhead.net/.
The coffee shop has really great views of the lake and you can sketch all you want and have hot apple cider or coffee. It’s a good spot to bring a little watercolor kit. But I usually go there to sketch the people. The image at the top of this post and the one above were all done at the coffee shop. There is also a little kids park right by McKenzie's Waterski School (all in the village). You can set up by the flagpole and get some good views of the lake. Some pictures of the village: http://lakearrowheadvillage.com/Merchant5/merchant.mvc?Screen=CTGY&Store_Code=LAV&Category_Code=PHOTOGALLERY
and here's some live web cab views of the area including the village: http://www.mountaininfo.com/webcams.html
The Village itself is also fun to sketch.
But the real attraction up here is all the great places for plein air painting. Pretty much anywhere. If you want a day trip to paint a lake, Lake Gregory in Crestline is perfect. You can get right on the shore here and in the winter it’s really cool to paint it with snow. Plus it’s a lot lower than Lake Arrowhead so a shorter drive. But any of the areas up here are prone to snow in the winter, so sometimes chains are required. Caltrans lists conditions here: http://rimoftheworld.net/caltrans