Here are a few series i created, all of them inspired by my daily life :)
1) Nail polish series
I have a huge collection of nail polish (that i don't even use :p) so i decided to draw my favorites with a pen. I then added a touch of color with watercolors. No special restrictions for this one.
2) Chairs from my house
Restrictions : 10 minutes countdown + i had to draw every type of chair in my house, even the weird-looking ones :D
I went from room to room with a piece of paper and a pen; didn't care much about perspective.
3) Digital series : favorite books
Proof that your drawing skills don't matter -- I sketched super quickly the covers of some of my favorite books, and ta-daaaa : a nice-looking drawing series :)
Restrictions : no details, just copying each cover and writing down the title + author's name