I agree with thorntree, Mizzy Lake trail and also Centenial Ridges are wonderful day hiking trails.
One note with Centenial Ridges is to allow lots of time to complete the trail most people take between 4-6 hours to complete it, depending on how much they stop to smell the flowers or take in the view. A short option to Centenial Ridges is to take the right fork half way up the hill and follow the trail counter clockwise this takes you to the best look out point in under 40 minutes. That said you'll miss one of the best trails in the park just going to the lookout point but if you don't have half a day to hike it's an option.
I would also suggest Booth's Rock trail at Rock Lake, this is a shorter trail as compaired to Centenial Ridges it also has a fantastic high cliff look out and follows the old railway line along the lake side back to the parking lot.
You can see a google map showing all of the day hiking trails in Algonquin Park on the Algonquin Outfitters website.
Algonquin Park Day Hiking Trails - Google Map
Algonquin Park's list of Day Hiking Trails
* Before heading out on any trail, consider length and rating of trail, time of day, and weather conditions.
* Allow enough time to be back at your vehicle before dusk.
* During the "off-season", do NOT start out on any trail after 3:00 p.m.
* Wear proper footwear, and appropriate clothing for the time of year.
* Use CAUTION when walking trails. High winds and storms can result in damage. You may encounter downed trees, wet areas, snow, and icy conditions. Blue trail markers identify trail routes.
* Regular maintenance on all interpretive trails begins in May and continues through to the end of October.
* Cell phone coverage may be "limited" or "not available."
* Guide booklets are removed from trailhead dispensers at the end of October but are availabe online, and continue to be available year-round during business hours at the East and West Gate and the Visitor Centre.
Your safety in Algonquin Park is ultimately your responsibility — be prepared!
(at km 7.2) This trail is a 2.1 km loop leading along the Oxtongue River to scenic Whiskey Rapids. The trail guide discusses the ecology and history of an Algonquin river.
Hardwood Lookout (**currently closed**)
(at km 13.8) This 0.8 km walk introduces you to the ecology of a typical Algonquin hardwood forest and culminates in a fine view of Smoke Lake and the surrounding hills.
(at km 15.4) This 11 km trail requires an early start and a full day to do properly. It visits nine ponds and small lakes and affords some of the best chances to see wildlife in the Parkway Corridor.
(at km 19.2) The Peck Lake Trail is 1.9 km long and goes completely around the shoreline of Peck Lake. The trail guide explores the ecology of a typical Algonquin lake.
Track & Tower
(at km 25) A 7.7 km loop featuring a spectacular lookout over Cache Lake, this trail introduces you to some fascinating history. A 5.5 km optional side trip follows an abandoned railway to Mew Lake.
(at km 27.2) This 3.5 km loop leads through mixed forest to an impressive view of Jack Lake. The guide presents results of research in Algonquin.
(at km 30) The 5.5 km of this trail passes through hardwood and coniferous forest including a large hemlock grove. General Algonquin ecology is discussed in the trail guide.
(at km 31) This 2 km loop includes an easy ascent to a pine-clad cliff and introduces the importance of change in the natural forests of Algonquin.
(2 km south from km 37.6) This very demanding 10 km loop affords spectacular viewing along two high ridges. The guide discusses contributions of selected people to the Park over its first century.
(at km 39.7) This 1.9 km loop is a fairly steep and rugged trail which rewards the hiker with a magnificent view of several hundred square kilometres of Algonquin. The trail guide discusses the geology of the Park.
(8 km south from km 40.3) This 5.1 km loop visits two lakes and a spectacular lookout, returning via an abandoned railway while the guide discusses human impact on the Park.
(at km 40.3) This 2.9 km loop visits about 80 huge, old White Pines and the site of an 1880s logging camp. The guide discusses how these magnificent trees came to be there, pine ecology, and the Park's logging history.
(at km 42.5)
Several boardwalk sections in this 1.5 km loop give you an excellent close-up look of two typical northern spruce bogs. The guide discusses their ecology.
(at km 45.2) A 2 km loop yields excellent views of two beaver ponds while the guide provides an introduction to Algonquin's fascinating beaver ecology.
(at Achray Campground) on the Park's East Side This beautiful 4.5 km trail loops through Algonquin's pine forests so typical on the Park's east side. The trail guide introduces you to pine forest ecology.
(11 km past Sand Lake Gate) on the Park's East Side This 1.4 km loop leads along the rim of the Park's most breathtaking river gorge. The history and ecology of the canyon is discussed in the trail guide.
(32 km from Hwy 17 on Brent Road) on the North end of the Park From an observation tower, this 2 km loop explores the eroded wall of an ancient meteorite crater. The history and ecology of the crater is the theme of the trail guide.