The green one (on my wife's hand for scale) was found swimming in a rockpool, looks a little like a sea slater but not any in my admittedly limited guides. The other was crawling down the beach (it was about 20mm long). It liked to keep its tail tucked underneath its body, possibly a young larger crustacean?
The one presenting its rear view is probably Talitrus saltator, the Sand-Hopper. They're so numerous around here that I remember one night when I literally heard them seething on the sand
The green one could be an isopod called Idotea baltica. The ID books often show them as being brown, but they can be green as well. I wouldn't have recognised yours from my books either, but I chanced upon photos of green ones like yours on the net. I'll be interested to know if it's the right ID.