spinach, kale (just the leafy part), celery, carrots. All blend well with various fruits. Add ginger root or lemon for something different.

Nutritionally, do you think the lettuce is worth is?

Romaine or other lettuce is pretty easy
Cucumber
Chard is softer than kale, but the taste may be stronger
Also maybe try removing the stalks of the kale before adding it
Parsley

Avocado. Though technically it's a fruit. Or you could juice the vegetables and then add the juice to the smoothie.

I usually just use frozen spinach + fruit. I've tried kale and beets but they were too fibrous and didn't blend well.