Very nice. I like the penning a lot. Your style is cute. <3
I've been wanting to get into ATCs recently. I should make some up, with a paper cutter or something. Heh.
Also, I love the heart sign. Very nice. I've been wanting to draw someone doing that for a while, too. Haha.

yeah, I find bristol is the best ATC material because it's strong and can take a beatin'. :)

Though, that having been said, I only do colored pencils on the bristol- I haven't tried watercolors yet, but marker bleeds. D:

Yeah, I've done some work with all three of those on bristol.

Watercolor is okay as long as you use it very sparingly. As in--don't take big gobs of water & paint and put it on the bristol and expect it to work.
Markers sometimes do bleed, but I find it the second-best next to real marker paper.
And pencils are so nice on bristol. I love how it's so smooth and the textures are totally up to you and your pencil strokes, as opposed to sketch paper which adds unwanted textures.