Jotunheimen means "land of the giants" which is due to the fact that this is the region of norway with the highest mountains, reaching 2469 m with the mountain "Galdhoppigen". A great but also very strenous and popular hiking trail starts at Gjendesheim (Maurvangen). The first part is really a hard climb from 1000 m at the lake up to 1743 at the highest point of the tour. That'll certainly take you 2-3 hours depending on your shape. But the spectacular view will reward you...views of the mountains and lakes of Jotunheimen with Gjende lake directly below you...this is how a bird must feel! From now on it's a small but steady decline until you reach the Beseggen ridge, a small ridge leading you down to the lake Bessvatnet. This part of the trail is very steep in places so watch your steps. Also, if you're afraid of heights don't do it since the drop to both sides of the ridge is quite dramatic.
Once you're down at the lake it's another two or three hours down to Memurubu, a mountain hut right at the lake Gjende. From there you can take the ferry back to Gjendesheim. Make sure that you have the schedule before you start hiking so you know when to be there to catch the last boat. Otherwise you're screwed and either have to stay in the hut (if there is space for you) or hike back. There is actually another trail on the lakeside but it'll nevertheless take you 3-4 hours to Gjendesheim.
Another option is to take the ferry in the morning and hike the trail back to Gjendesheim, however, you would then have to hike UP Beseggen ridge. This will not give you the permanent view of the lake Gjende since you'll have it in your back. Most people therefore hike it the way I described it above. Be also alert that especially on weekends the trail can be heavily overcrowed since it is the most popular trail of norway and almost every norwegian have to do it once in his (or her) life.