More formal:
* Collecting refuse
* Gathering litter
* Removing debris
* Cleaning up trash
* Waste disposal
* Waste management
More informal:
* Picking up litter
* Cleaning up the mess
* Scooping up trash
* Getting rid of garbage
* Tidying up
More specific:
* Bagging trash
* Sweeping up debris
* Collecting recyclables
The best phrase to use will depend on the specific situation and who you are speaking to.