We will be visiting relatives in Green Bay and have set aside one day to do a day trip. We have already been to Door County, canoeing on the Brule River, and down to Milwaukee and Cedarburg. We've also spent lots of time in Madison.

I really prefer places where we can get deep into nature. Or else really cute towns. Off the beaten track is fine. Preferably within 3 hours of Green Bay. Any suggestions?

We have friends with a second home in Green Lake, and they like that area, and the neighboring town of Princeton:
Nestled on the banks of the Fox River, Princeton provides a myriad of diversions for its visitors. The area is a haven for those looking to relax and unwind in a pleasant, peaceful environment. Rich in natural resources, Princeton provides a bonanza for the outdoor enthusiast who will find Princeton and its surrounding areas abundant with natural resources, streams, and lovely landscapes. Fishing, canoeing, cycling, camping, and many other forms of outdoor recreation are readily available for the naturalist. In addition, Princeton boasts a downtown filled with eye-catching architecture and rich history. There are many delightful shops and restaurants where the visitor can while the day away. Princeton is minutes away from Green Lake, Lake Puckaway, and White Lake, and just a short drive on picturesque country roads from dozens of other lakes and trout streams.

Thanks! Sounds great. Anyone else?

What time of yesr will you be here(are we talking snowshoeing or kayaking?)?

Early April.

Waterfalls of The Western U.P.

"There are more than 150 waferfalls across this breathtaking area of the Upper Peninsula"

http://www.westernup.com/waterfalls/

For the true Wisconsin nature experience, you need to head "up north". Within a couple of hours of Green Bay, you're in Nicolet National Forest. There are lakes and streams and trails where you'll see no humans for hours. Hayward is a town that comes alive in the summer-think lumberjack shows and knotty pine knicknacks-but has any services you'd need in April, which is likely to be cold and damp. Green Bay is only a couple of hours from Marinette, Michigan, thus the UP references.
My "Wisconsin Wildlife Viewing Guide" describes some areas close to Green Bay:
"The early springtime spawning of lake sturgeon is an exciting event not to be missed. These prehistoric giants have survived to modern times and annually swim upstream from Lake Winnebago to spawn on the rocky shorelines of the Wolf and Ambarrass rivers". They are visible from the DNR Mukwa Wildlife Area near New London (CC#414- 982-5822 or Wis DNR).
Also migrating that time of year are all the waterbirds; tundra swans arrive first.
Bishcoff Road Viewing Site near Sciocton. (Wis DNR 414-832-1804)
Navarino Wildlife Area has trails to see the swans, sandhill cranes, beaver and otter and more; grassland and boardwalk trails and a nature center(715-524-2297).
Have you been to Bay Beach Wildlife Sanctuary in Green Bay?
Just below Door County is Point Beach State Forest; woods and beach and a lighthouse. Continue down I43 along the lake to Manitowoc and Sheboygan. I love the drive above that, too, through Algoma and Kewaunee. Sheboygan is really undergoing a renaissance and has some amazing restaurants and shops along with its free John Michael Kohler Art Center and the riverfront and lakefront. Kohler, just west of Sheboygan, has shops, restaurants and the Kohler Design Center.
Kohler-Andrae State Park is also on the beach and has walking trails and a nature center. You'll see the waterbirds migrating and trip over deer.

Early April --- the ice won't be off the "up north" lakes and it's very possible that we'll still have plenty of snow.
If you enjoy winter activities check the Wisconsin DNR website. www.dnr.state.wi.us
You'll find many state parks within 3 hours of Green Bay.

Thanks everyone! I'll definitely print this, as we visit quite a bit, and we'll never be able to fit all this into one trip.

kp is right-early April is winter, not spring, in Wisconsin. Who am I kidding? We don't often have spring in Wisconsin. For warmer stuff, you have to head inland and south, but if you ski, you'll still be able to do that north of Green Bay.