Well you hit on a top one for me, reading. I don't know if the nook lets you download from other sites, but I'd suggest the free books from Project Gutenberg.

Then there's always the puzzle books (the App Store has a free logic puzzle app), coloring, knitting and crochet, counted cross stitch, embroidery, even the latch hook kits that make 70's shag carpet pillow things. Tv and movies.

But writing can be fun too, you don't an English degree or even too much of an imagination, when I'm really bored and feeling less than inspired to come up with my own ideas, I like to write alternate endings to books I've read or movies I've seen, even rewriting lyrics in my head. Silly stuff really...

Poetry is also fun to write, be it simple rhymes

Ode to soda: fizzle, fizzle soda pop / bubbles bursting from your top

or even haiku... 5-7-5 the number of syllables per line.
Like: A fat cat sitting / sunning in the window sash / watches birds fly by.....

They can be fun, and a great outlet. Though I will say haikus are also probably the easiest ones to share with people for some added fun without the embarrassment of critique, worse someone will say on those is you counted your syllables wrong