My husband built a sandbox for our daughter that has provided her endless hours of entertainment. It was relatively inexpensive and easy to make. Sand is very cheap so we replace it often.
If you aren't bothered by a mess I would also suggest finger paints. We have an inexpensive plastic table outside that I allow our daughter to paint on. I literally strip her down to her panties and she will paint for hours.
You may already be doing this but I found that engaging my daughter into everyday chores is fun for her. She loves to help sort the laundry, wash the dishes, dust, etc. Even though her "help" is not necessarily accretive she finds these chores very pleasurable. When I cook I allow her to pour the ingrediants in after I have measured them.
My last suggestion is to try hiding letters. We have those foam bath time letters. It will be fun for her to find them and a great learning experience since you can talk about the letters.