Hi, Super Pickle mama. I would like to echo what Elismama said. I think of my job as two part: (1) fueling and (2) getting out of their way.
Fueling means many things. I provide good raw materials (sand, dress-up collection, paint/etc, small and large blocks, etc). WHen I was little my parents got a few light wood blocks which were huge and painted them bright colors; we could make forts and whatever! I am in the process of making something similar for my little ones using sturdy wine boxes (Carlo Rossi ones are strongest!) I also provide non-directing attention, for instance echoing their creative constructions like "Oh, that's a phone? Hello!" or role-playing. This was a lot more work with #1; my toddler #2 mostly plays imaginatively with his big sis. Toddler imaginative play tends to be repetitive, because they learn by repetition; I remember playing "babies on a boat" with #1 about a million times. Really showing up and giving myself to my children's play is a gift that lights them up. (These days my 4 1/2 year old still loves to pretend I'm her less-experienced playmate on the play-structure, so I'm the only mama at the park using a baby voice...)
Getting out of their way means being on the look out for ways in which adult ideas might impose themselves on my children's imaginations. I avoid TV. Like Elismama said, I encourage creative use of toys rather than "intended," and favor toys which have no one "intended" use. And similarly I avoid adult-designed crafts like the plague, wanting to prolong as long as possible my children's belief that they are the master of their own materials. This tends to be a bit more boring for the parent (?), at least, my friends tend to look to more advanced/packaged toys and activities so that they themselves are a bit more entertained (SAHM-ing can be a marathon) but I find it very entertaining to see what my children think of next, and as I say, it's so much easier now with two!
Also, I keep very few toys out at any given time, so they make the most of a few key ones, and they look less to things and more to themselves for novelty. It's also less clean-up.
Have fun, Super Pickle! Again, I know that I am not saying anything new, just wanted to chime in because this is one of my favorite topics!