I put the brown ones (no need to soak overnight) in with braising steak in the slow cooker. Makes the braising steak go a lot further.
Add some onion and carrot and a veg stock cube, switch on and go out for the day.
You need quite a bit of water if putting lentils in slow cooker though to make sure it doesn't dry up when they absorb the liquid.
woodstock that lentil and feta salad sounds fab - but not enough details!!! Questions: what kind of lentils (brown, orange) do you just boil them up and then bung dressing and feta on?