I was at both Olympic and Rainier back in 2000. My in-laws travelled with me and my wife, so I didn't get to hike as much as would have liked.
Rainier: Sadly, I didn't get to hike here much at all, but it was for a particularly good reason. This is a massively large park, and there are a whole lot of things that can be seen from the road and short hikes. I did one high alpine hike right at the tree line in the Paradise area, and it was really wonderful - wildflowers were in bloom everywhere. If I recall correctly, the Paradise visitor's center is the starting point for many good hikes, including hikes to some of the glaciers. In my opinion, the best views of Rainier are in the Sunrise area. If I ever go back, I definitely want to spend more time there. One thing that you might want to take into account is that it is always cool and damp around the mountain. That part of Washington gets a lot of precipitation, and at high altitudes is can get quite cool. We went in August, and I took only shorts expecting it to be really warm - a really bad mistake. Since you are a photographer, make sure you take advantage of any clear weather around the mountain - during the week we were there the top of the mountain was clear of cloud cover sometimes only minutes each day.
Olympic: First and foremost: give the rainforest a chance. You don't have to spend a lot of time there, but it is definitely worth a look. At the Hoh Rainforest visitor's center there is a short walk through the rainforest that is worth the time. This may sound kind of stupid, but on this hike you will see a creek that has the clearest cleanest water I have ever seen - if not for the ripples on top of the water, the water itself is nearly invisible. Once again, we didn't do a whole lot of long hiking here, but there are some really nice waterfalls that are must-sees (most notably, Sol Duc). Hurricane Ridge is a really scenic drive with lots of wildlife (herds of deer that are not real afraid of people). Also, the coastal areas of the park are really nice. In particular, we went to Rialto Beach and I hiked to "Hole-in-the-wall", a natural sea arch. Around this arch (particularly on its far side) are some really nice tidal pools filled with all sorts of creepy-crawly sea things. We also spent a day in the Kalaloch area, which is more really nice coastal scenery.