Don’t know if you can get in to Blue Lagoon at night, but if you can’t and have to go during the day, go for the earliest possible time slot. It gets mega busy from the second slot onwards.
Didn’t find it too expensive. Like drinking in bars is about the same price as any UK city centre and food is reasonable too. Buying drink out a supermarket is $$$$$. Get some booze at the duty free on your way out of the airport.
Don’t know if it’s in your budget, but we hired a car and found that much easier for getting around. If you do the golden circle on a bus tour then you only have a certain amount of time at each site and then have to be back on the bus, if you have your own car you can go at your own pace. We also drove down to Vik for a day and that’s a lovely drive, you go past some other cool waterfalls and get to see Eyjafjallajökull which is the volcano that messed up everyone’s flights (there’s a wee visitor centre there too).
Eat at this hot dog stand:

Iceland is amazing man, enjoy yourself!