Splashy walk with wellies, waterproofs and umbrellas? Always makes my boys in a better mood, and will concentrate on something (rather than beating each other up!) You could go armed with a list of things for them to find/spot.
Alternatively / additionally: make some buns (on your own, or with boys taking turns). Then ice/decorate them, or buy (if funds permit) some of those small tubes of writing icing. Another thing that my 2 enjoy - perhaps as we don't do it very often.
Get the 2 older ones to write a letter to a friend or relative, younger 1 draws a picture. Then off to post box (you never know they may get a reply!)
Are these any good? Reading back they seem a bit old-fashioned...