For my 2 1/2 year old, I do make a game out of it. I ask if he can help me find the "red toy" or the "green truck";etc. He's excited to find it and he's learning his colors at the same time. We do it together, so it's not so overhwhelming to him. He's not always cooperative, so there are times I say it's mine, pick it up, and put it out of his reach (he can see it, but can't get to it). We come back at the end of the day and put it away so we can play with it the next day. As for the 5 year old, I expect his room to stay clean and give him the responsibility of informing his friends the room needs to stay clean. If he doesn't speak up to his friends to help him clean after playing, it's his responsiblity to clean after they leave. It's takes some repeating at times, but I inform the 5 year old if I find this particular toy on the floor in the next 5 minutes, it's going to the donation box---he moves quickly.
Second, if you are constantly seeing toys on the floor, you may need to donate toys on a regular basis to a women's shelter or an orphanage (we have an orphanage near our home). Four times a year we donate--I have a box (out of view of the boys) and as I notice toys at the bottom of the toybox no longer being played with, I clean them and put them in the box. Once the box is full, it's taped and ready for delivery. If you are dealing with less toys, there's less mess to clean up. I do explain to my five year old how fortunate he is to get as many toys as he does at Christmas and birthdaytime. He's willing to part with toys before Christmas and his birthday to make room for the new ones coming. This makes for less clutter while teaching the 5 year old about doing good things for others (a great life lesson). Hope this helps!