I’ll add my two cents as a former local. Lots of options to fill out your trip. Here are some ideas:
Other zoos/aquariums
The biggest of the non-AZA zoos are Hemker and Safari North, both in central MN. Both are privately owned zoos so the setup is basic paddocks and wire cages. As mentioned in the post above there are some rarities to be had at both. You might consider a visit if there’s a species you particularly want to see, but I’d personally pick other options ahead of them.
Also as others have mentioned, Sealife and Great Lakes aquarium are the two aquariums to consider. Sealife is bigger/better than the typical mall/tourist trap aquarium in that it has four big tunnel tanks (N. American freshwater, Amazon, Sharks/Jacks/etc., Coral Reef) instead of one. In terms of species the most interesting are probably Bowmouth Guitarfish and Green Sawfish. If you’ve been to a few bigger aquariums you’ve probably seen everything before though.
Great Lakes aquarium isn’t huge but is more unique. You just have to go knowing that the star attractions are otter, sturgeon, and trout. There are usually some rarely exhibited fish on display. I’ve not visited for quite a few years but in the past recall seeing Arctic Grayling, Deepwater Sculpin, and Iowa Darter, among others. If I was visiting Minnesota and wanted to add one additional zoological attraction I’d pick this one. Very easy to combine with the Lake Superior Zoo, as well as some nearby outdoors destinations.
Wildlife/Parks
Among charismatic wildlife species, Black Bears are 100% guaranteed with point-blank viewing/photography opportunities at the Vince Shute Wildlife Sanctuary, which is a couple hours north of Duluth. I’d strongly consider a trip here if you don’t mind viewing wildlife at a site where a feeding station has been set up. It could be combined with some of the parks north of Duluth. For example, driving the roads in Superior National Forest gives an outside shot at Moose. Highway 1 and its offshoots are a decent option for this. Note that the road that goes straight north from Duluth, Highway 53, services huge iron mines and isn’t good for wildlife.
Loons and Bald Eagles are both abundant in Minnesota, especially in the north. If you want to see them around the Twin Cities, spend a bit of time along the rivers for eagles, and go to Vadnais Lake for loons. You'd likely see both if you spent time around Duluth also. Other charismatic species are either too elusive (Wolves, Lynx) or too far from the places you'll be (Elk, Bison) to reasonably target on a short trip.
If you’re into birding, McGregor Marsh (between Duluth and the Twin Cities) is the best place in the world to try for Yellow Rail. The Sax-Zim bog near Duluth is known as a winter spot for owls and boreal songbirds, but even in summer is good for specialty birds like Connecticut Warbler and Black-billed Magpie.
For parks, a good option right in the Twin Cities is Fort Snelling State Park. Interesting historic fort, and surprisingly good for wildlife. Eagles are pretty much guaranteed, and the river backwaters are good for beavers. A little farther afield is Interstate State Park, which has some interesting glacial geology and again is a good spot to see eagles. North of Duluth the nicest park is Tettegouche State Park, which has large waterfalls, cliffs, extensive forests, and some interesting smaller wildlife like Franklin’s Ground Squirrel and Snowshoe Hare.
Cultural Attractions
Most of the major cultural attractions are in the Twin Cities. If you like art the Minneapolis Institute of Arts has a good collection with works by the likes of Rembrandt, Van Gogh, etc. and is free. There are two museums with natural history elements – the Bell Museum and the Science Museum of Minnesota. Both are nice but I’d personally skip them on a short trip. The Mall of America is obviously an option also. Minneapolis and Saint Paul each have various dining/shopping districts too but I’d skip these on a zoo-focused trip.
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