Went in May, which is like an off-season. Best time to go is July-now if u wanna see everything, in winter many places are inaccessible but u get to see the northern lights.
I went there F&E, then booked those bus tours to places of interests. There're tons of websites of pte tours u can book in advance.
Driving is obviously the best choice but u must be confident in driving a 4x4 in very rough terrain (if u intend to off-road).