The "clean up" song always worked for my children. When they are just wee lil' tots you will need to help them pick up their toys to set an example so they know how you expect it to be done. You will probably need to do this with them for quite a few times and then slowly back off and make them do the majority until they can do it all by themselves.
You can also limit the number of toys they are allowed to have out at any one time. This will minimize the amount of toys needing to be picked up at the end of play time and make it less overwhelming for tots.
Also, there is no harm in telling your tot that there will be no cartoons or no going outdoors or whatever they are wanting to do next or whatever is their favorite thing until they pick up their toys. And you should word it so that they are making the choice and not you just ordering them to do something. Tots like to make their own choices, just be specific and limit it to two items to choose. Like, you can pick up your toys or sit in time out, which one do you want to do? Or, you can pick up your toys and have pudding for snack or sit in time out and have crackers. Don't take food away as punishment but offer something less desirable.