I'm lazy and usually use old fashioned, but steel cut definitely have the better texture to them. Suggestions for add-ins:
applesauce...possibly with cinnamon and raisins
fruit (fresh, dried, or frozen)
chopped nuts or seeds
whey protein powder
flaxseed or wheat germ
peanut butter...or PB and pureed banana
trail mix
chopped up Larabar (or similar energy bar)
vanilla extract, cinnamon, ginger, or cocoa powder with sugar (or sugar sub)
honey, brown sugar, maple syrup, or other sweetener
pumpkin, sweet potato, or butternut squash puree
chili powder
grated parmesan cheese or cottage cheese
soy sauce
eggs or egg whites
yogurt (flavored or not, your choice)
Grape Nuts or some other cereal or granola
coconut
chocolate chips, butter, pie filling, m&m's, or nutella if you're feeling indulgent
cocoa powder or chocolate whey, cinnamon, vanilla extract, milk, brown sugar, and maybe a dash of chili powder for Mexican cocoa oatmeal
crushed graham crackers, mini marshmallows, and chocolate chips for s'more oatmeal
Silk Nog, nutmeg, chopped pecans, and crushed candy canes for holiday oatmeal
cooked with milk (or chocolate milk), tea, coffee, or chicken broth in place of water
...or with a dash of Bailey's

also:
oven baked oatmeal
oatmeal pancakes
oatmeal cookies
smoothie ingredient
in chili or tomato soup (rolled might work better for this though)