Here is a maths worksheet on addition for year 5 which challenges children to use all the mental methods of addition that they have developed.
One of the key features of adding mentally is that it is usually done in a different order than when using pencil and paper methods. When adding two 3-digit numbers it is usually much easier to start with the hundreds, then add on the tens and finish with the units: the opposite of standard written addition. Also remember to look for pairs of numbers that make 10 or 100, especially when adding three numbers.
This page is not as easy as it looks!