The easiest thing to stack is her blocks. We have crappy all-purpose-y style cheapo apartment carpet, installed over a carelessly uneven and thin pad, so we have what just might be the least stable base for stacking a vertical tower. Charlotte has an amazing patience for it, and will stack and restack without end. She also stacks things on horizontal surfaces - the coffee table, her toy kitchen, upside-down boxes, the corner of the kitchen table she can reach, and her closed potty.
Sometimes there is the double-whammy of OCD toddler behavior - the stacked tower and the line up of toys on the floor. Honestly, if I didn't have as much experience with toddlers as I do, and haven't researched so much about things as I have, the stacking and lining up would make me a wee bit nervous, autism spectrum wise, but I'm pretty confident on that front.
Charlotte's favorite stacking toys, the absolute ultimate in compulsion, are the blocks from a Haba set (similar to this set, but without the car - man, she would like that car) that her Aunt H got her for her first birthday. She's liked the set from day one, but the solid feel, stability, and the intrinsic set-ness of these blocks make them Charlotte's favorite stackers. Literally the first thing she plays with in the morning is this set, and the last thing at night. In fact, starting tonight, we took them out of her room at bedtime, since the last three nights she's gotten out of bed to stack them immediately behind the closed door.