An open-ended investigation experiment involves trying lots of situations with only one factor changed each time, all aiming to answer the same question. If you want to find out "what can keep ice from melting?" your general question could be rewritten to ask "In what ways might I keep ice from melting?"
With that general question as your starting point, you can begin to create experiments to test various answers. If you put the ice in a cardboard box on the kitchen counter, will it melt? If you put the ice in a cardboard box in the freezer, will it melt? If you put the ice in a cooler on the counter, will it melt? If you put the ice in the cooler in the freezer, will it melt?
After testing a variety of situations, you should be able to look at your test results and draw some conclusions regarding circumstances that would prevent the ice from melting.