You can remove the stain from the gravel the same way you clean the driveway, with muratic acid sprayed from a pump sprayer. Im not sure of your budget constraints on this, because the dimensions of the playground is not given.
That would depend on the type of "cleaning" your doing
It is easy with the right tools, a magnetic cleaning pad for the glass but make sure there are no stones stuck to it or it will scratch the glass. Also a gravel cleaning suction device, this will clean your gravel during weekly part water changes.
Mr. Clean
Use a piece of window screen put pea gravel on it and wash with a garden hose till it is clean. good cleaning.
small amount of dish liquid
Many cleaning products do such as when comet and windex mix and the most dangerous cleaning product is mr clean
soak it in coke or pepsi
Windex is great for cleaning scanners.
Cleaning kits that come with cleaning solution and scratch free cloths are best. Covers will help you protect your PSP and help you keep it clean.
The best way to clean leather cellphone covers is to use a cleaning product like Mr. Clean Magic Eraser. Make sure to be gentle when cleaning the cover as over cleaning leather tends to darken leather.
There are a lot of product brands that can help clean ones pores. Clean & Clear and Bior̩ are considered to be the most reputable products for cleaning pores.
Resolve is a very good carpet cleaning product. Another good one is BlueMagic 912.