It is all your style. You want the pictures to reflect YOU. We are getting FI's mom to take ours (she has photography experience), and went to take some during the summer (we are taking more in the fall). We found out that we laughed the whole time and just joked around with each other, teasing and everything. It was silly and very us and I love the pictures! At the same time, we also love being dressed up, so we are going to be a little more stylish mixed in with fun next time.
I guess what I am trying to say is that it really depends on you as a couple and your location. We are going to play in a park with our dogs when we go next time, which is very typical for us. If you are a very sophisticated style, goofy playing in leaves pictures may not suit you. Similarly, if you live in sweats and jerseys most of the time, cocktail dresses, suits and heels will look out of place.
My best recommendation is to try and find some middle-of-the-road outfits. I would google tons of pictures for ideas, and especially look at what your photographer has done before. Let's say you can do 3 outfits. I would do:
1 very nice, but casual, look: Dark denim, nice sweater or blouse.
1 dressier look: little dress, heels, that idea.
1 casual dress OR more casual (another colour is a good idea) OR maybe a jersey or something if you are avid sports fans?
For our summer ones, I wore layered tanks over a denim skirt. For the fall ones, either a black or deep pink sweater over dark denim. I am also bringing a jacket I love so I look more put-together but can take it off too. Layering might be a good idea.
Okay, that is a lot of layering. My advice, in summary: bring different type of outfits. Google LOTS to get ideas of what you love for inspiration.