To compare some greens:
Nori
Laver, 1/8 cup (2 Tbsp)
Calories: 3.5
Protein: 0.58g
Carbohydrate: 0.51g
Total Fat: 0.028g
Fiber: 0.03g
Spinach
Spinach, 1 cup (50g) (raw)
Calories: 6.6
Protein: 0.86g
Carbohydrate: 1.0g
Total Fat: 0.105g
Fiber: 0.810g
*Excellent source of: Vitamin A (2,014 IU)
*Good source of: Vitamin C (8.4mg), and Folate (58mcg)
Romaine
Romaine lettuce, 1/2 cup (35g) (raw, shredded)
Calories: 3.9
Protein: 0.45g
Carbohydrate: 0.66g
Total Fat: 0.056g
Fiber: 0.47g
*Good source of: Vitamin C (6.7mg), and Vitamin A (728 IU)
Iceberg
Iceberg lettuce (raw, shredded), 1 cup (56g)
Calories: 6.6
Protein: 0.55g
Carbohydrate: 1.15g
Total Fat: 0.104g
Fiber: 0.77g
Red leaf
Red leaf lettuce, 1/2 cup (28g) (raw, shredded)
Calories: 5
Protein: 0.36g
Carbohydrate: 0.98g
Total Fat: 0.08g
Fiber: 0.53g
*Good source of: Vitamin A (532 IU)
Bok Choy
Bok choy, 3 oz. (85g)
Calories: 14
Protein: 1.0g
Carbohydrate: 2.73g
Total Fat: 0.18g
Fiber: 0.84g
These are not all the same suggested serving sizes, unfortunately. However, it is very clear that nori has more calories, protien, and carbohydrates than lettuces. It also has less fiber. But I'd think that there are many vegetables that could work if you try not to overdo any one nutritional aspect. For instance, broccoli florets.
Broccoli
Broccoli (raw), 1 cup (133g)
Calories: 25
Protein: 2.6g
Carbohydrate: 4.6g
Total Fat: 0.31g
Fiber: 2.6g
*Excellent source of: Vitamin C (82mg), and Vitamin A (1,357 IU)
*Good source of: Folate (62.5mcg)
Here is where I just checked the comparative nutritional values:
http://www.healthwell.com/healthnotes/healthnotes.cfm?ContentID=1987000#Food-Contents-List