Hi C.!
I am a mother of 3. My son is 4 1/2 and has a very vivid imagination (maybe too much sometimes). He does a great job playing on his own, although his attention span is not always very long. And he is also fortunate enough to have two sisters (2 yrs and 10 months) to also keep him occupied. I have listed some of the toys/games that he uses a lot of imagination with below.
kithen - I got my 3 kids a play kitchen this past Christmas. It was mainly for the girls, but my son plays with it a lot too. He makes entire meals and then brings us the 'play food' to eat.
tools/yard equipment - Probably the best $12 dollars I spent was buying him a play lawnmower when he was about 18 months. He still uses it - especially during the summer. He also loves the Home Depot line of toys (from ToysRUS). He has the weedwacker and blower. Every time my husband mows, my son gets out all of his equipment too.
building toys - My son loves to build things. We got him the large cardboard building blocks when he was about 1. He still uses them to build towers, fences, beds, etc. He also likes brix (sp?) blocks, tinker toys and lincoln logs. He also really enjoys playdough. A lot of times while I am making dinner, I will get out the playdough and let the 2 older kids play at the table (where I can watch them). They like to use cookie cutters with it and my son likes to build roads for his cars out of playdough. During the winter I would bring in large cups of snow and let them 'build' small snow men at the table. They got a little wet, but had great fun.
dress up clothes - My son has a few dress up outfits - darth vadar, pirate (old halloween costume) and a knight shield/sword. He also likes to use his dr kit. He will come up and give us shots or take our blood pressure. And he sometimes borrows my daughter's dress up clothes (but dont' tell his dad).
play figures - When he was younger he enjoyed the little people sets. Now that he is older, he likes the playmobil sets or other action figures. He got a Playmobil safari set for his 4th b-day with lots of animals. I then got out an old safari 'Adventure People' set I had from when I was young and it complimented his set well.
Sorry this is so long - but I was able to think of a lot of pretend play ideas... Hope this helps.