If you need a way to mark your lawn or garden that's cheap and won't interfere with a mower, try something like this:
Then you can just push it into the ground to use as a marker.
We were looking for some way to mark where in our lawn we wanted to set our sprinklers so we don't have to figure out where to place them every time we set them out. Flags would be in the way when we mow but plain stakes or nails would be hard to spot when they're pushed all the way down. So my mom came up with the idea to use the milk caps and a nail - since they're brightly colored they show up in the lawn but they're flat so they won't get in the way.