This is a highly recommended side trip on your way to the Kenai Peninsula or a great way to end your Sunday drive down Turnagain Arm.
Along the Portage Road, there are several glaciers with short easy hikes to the base of the mountains from which they flow down. Middle glacier can actually be climbed to, as long as you're careful.
At the end of the road is Portage Lake and a wonderful Museum of the Chugach National Forest. There is a great bookstore inside as well.
Unfortunately, the glacier has receded to the point where it is no longer visible but there are cool icebergs floating in the lake.
If you have the time, take the interpretive hike along Moose Flats at the beginning of the park.
Although I've never done it; it looks like a great place to ride your bike with either road and/or trail riding.
Don't be fooled on your climb up to Middle Glacier. It is steep, and when you hit snow, very slippery. As I was coming down I slipped on my butt and came way too close to smashing into a boulder. I wish I had bought some kind of ski pole or walking stick.
There is a federal campground on the premises; if you're lucky you can reserve a campsite with a picture perfect view of Middle Glacier. This is a really nice campground with a fair amount of privacy. For more information, go to Recreation.gov.
You can continue on through the tunnel to Whittier, "The Strangest Town in Alaska". The toll is $13 per vehicle, though you only pay going into Whittier, not leaving. It opens on the half hour from the Portage side, on the hour from the Whittier side. More information on the Whittier tunnel can be found here.
If you'd like to educate yourself about glaciers and their awesome power, please visit All About Glaciers, a great introductory website.