The Nano is nice but only 10 lumens. I like the K2 because it's around 25.
I know you said smaller than AAA but the Preon P0 $21 is a great little light I have. Its good looking and being all flood it's pretty useful to me. The light is just about AAA size and has a max brightness of 25lm and moonlight .24lm which I really like. Thrunite Ti2 is also a AAA light at about $11 with 3lm and 81lm. I've seen them for $8 at times. Again I know you said smaller than AAA but these are very slender and just about the size of a AAA.
Don't get a Maglite Solitaire. Since my Fenix E05 died (I'd get another, very good) I've been carrying the LED Solitaire. It hasn't stood up to being pocket carried at all well, it's got dings and scrapes all over and really looks like it's been through the wars over the past couple of months. The end also falls off on a regular basis (usually 3-4 times a week, twice today) meaning the battery falls out and it disappears from my key ring.