Located approximately 4 miles west of Moraine Park in Rocky Mountain National Park, these stunning lakes receive a lot of visitor pressure. The trail starts at the Fern Lake Trailhead and proceeds 4 miles to Fern Lake. Odessa Lake is 1.0 miles from Fern via the Fern Lake Trail and Spruce is 1.0 miles from Fern via the unimproved (I stress this word) Spruce Lake Trail.
We did this trip as a 3 day/ 2 night backpack trip. We didn't anticipate many problems getting to the lake, but we did underestimate the trail. The trail isn't overly difficult, but it is very rocky. The first 1.7 miles are a breeze to say the least, gaining only 350 feet of elevation. The next 2.3 miles will really test your lungs and legs, especially with a 50 pound pack on your back.
The trailhead was really busy for a Friday morning, but we had expected this. After the first 1.7 miles, you arrive at the Pool. This is the confluence of three streams, including the headwaters of the Big Thompson River. You cross the confluence on a nice foot bridge, and then follow the signs leading to Fern Lake (take the trail which leads up to the right). From here, the introduction is over. You climb relentlessly for another 1.0 mile until you reach Fern Falls. This is a dramatic waterfall, and the views from here are impressive to say the least. From here, it is another 1.3 miles or so to Fern Lake.
Once you have reached Fern Lake, Odessa Lake is 1.0 mile away. The trail to Odessa starts right at Fern Lake, and proceeds around the eastern side of Fern Lake. After 0.7 miles, you come to a fork in the trail. Take the right fork which will lead you to the northern shore of Odessa Lake. The left fork leads up the mountain and over to Lake Helene and back to Bear Lake. 
This trip was intended to be a fishing trip, and the fishing wasn't half bad. The lakes (all 4 of them, Loomis is the other), are all catch and release using artifical flies and lures only. These waters are Native Cutthroat recovery areas. The fish are pretty skiddish, and the slightest movements can spook them. We had decent luck on Friday evening, but the fishing really slowed on Saturday. I guess the fish know when the tourists are there!
To reach Spruce Lake from Fern Lake, go back about 0.25 miles to the junction of Spruce Lake Trail. Keep in mind this is an unimproved trail, and it is really rocky. There are several parts where there isn't much of a trail, and you have to look for rock cairns to reach the lake. For those adventerous souls, Loomis Lake is about 0.75 miles from Spruce, but there is no trail from what I understand. We were going to try to venture up to Loomis, but the weather looked less than inviting, so we didn't.
Overall, I would recommend this trip to anyone who has an extra day or a couple of days. It is fairly difficult to get a campsite reserved in the peak seasons, but the off seasons shouldn't be too bad.