Crestasee is my absolutely favorite lake and for that in any season. Summer is beautiful for swimming, sunbathing and having barbeque. Autumn and winter are uniquely peaceful for long forest walks or hikes. And spring is the season when nature is truly colorful. 
When you visit the lake I'd recommend you to not stop at the public picknick space but rather have your exploration walk around and then take rest in your favorite spot.
Directions:
By bus: Take the Regio line from Chur mainstation and hop off at Felsbach/ Crestasee. From there it is indicated and an easy 10-15 minutes walk takes you to the lake
By car: You have several options to park. The first parking (Trin Parking Nr.3) is the farthest away but you'll reach Crestasee in 40-50 Minutes.
Of course you can drive also to the next parking lot and there you'll only have 15-20 minutes to the lake.

Remember: the trail is very easily to follow :) 

Photography tips: Crestasee is very beautiful for sunrise or for sunset too (but make sure to arrive min. 2 hrs earlier to sunset because the lake is well hidden in the forest and therefore is earlier in the shade.).
The most prominent spot is definitely the footbridge but also if you continue from there on the right shore, you'll get many other lovely viewpoints with beautiful reflections. The old island with the tree lies also on that side.