Hi Ju can spent a few hours at the lake and than go to Masaya Market and back to Granada Everthing is very close it takes 1/2 hour to the Laguna from Granada and 20 minutes to go from the Laguna to Masaya wiht own car or taxi. If you take public transporation you can not make it in one day as there are only 3 buses per day.
The Laguna is not just a crater lake is THE CRATER LAKE and the most unique spot in whole of Nicaragua The water is year round between 25 to 27 degrees celcius. There are some hot spot on the shore where you can boil eggs but to small to seat into and even to hot..
Agree with Salta, there are no hot springs but it's an amazing crater lake, and it's the lowest point in all of Central America (at the lowest depth in the lake).
Consider a little time exploring the villages nearby that also offer views of the lake, like Diriomo and Catarina. You can also hit San Juan de Oriente.
For hot springs you would need to go to nearby Leon. They are boiling mud puddles called Los Fumaroles. It's fun and quaint (for now) but I wouldn't plan a trip just to see those unless you really like hot springs. Direct message me for more on the Fumaroles, I could share a photo with you, etc.
depends on how long you shop and eat for. Also depends on what form of transportation you use.
4 hours should be plenty at the laguna itself, just for swimming. But depending if you want to do some hiking, or go see the view from the top of the crater in catarina, etc. You may want more time. If you did leave at 2pm, you could make it to Masaya by 2:30, shop for 3 hours, eat for 1.5 hours and be in Granada 20 minutes later well before 8.
if you are going by public bus, add a little bit of time to the travel time. Maybe an extra 15-30 minutes each trip.
Yes there are hot springs at the crater lake, they are a bit away from the restaurants and lodges. On a public area but do not think they are develop as they are in Costa Rica, is hot water coming from ground and combining with sulfur water from the crater lake "amazing" bring some sandals to walk on the pebles.
I would say enjoy of the hotsprings as much as you can, have lunch in one of the many restaurants in the area go and enjoy of the two markets in masaya btw the regular market which also has souveniers is 10 or 15% cheaper. And then go and enjoy of the Lava tour in Masaya.
Give a call to http://golocalnicaragua.wixsite.com/nicaragua they can organize something nice for you.