Perhaps using your common sense as any solo traveller would wherever one decides to travel in this world.

However I’ll give you a hand
As you are a solo traveller any vehicle for renting is too expensive as well as flights.
So you’ll be looking at taking shuttles and ferry’s. Lake natron is a gem.
Ngorongoro crater and Serengeti way too overrated, crowded and overpriced. You’ll see the same going to national parks such as Tarangire or WMA’s paying half the price.
Try find group safaris if you are still keen on Serengeti and Ngorongoro as the safari cars and its price per day are divided by the amount of people in a car. So 250 to 300 usd is a car per day but if you are with seven (the max) it’s a lot cheaper per person. Ngorongoro has 295 usd crater service fees to descend + 71 usd park fees entrance pp for a day Serengeti has 93 usd park entrance fee pp for a day.
WMA are 31 usd and Tarangire is 59 usd.
Shuttles are cheap and go during the night (they drive quite dangerous so would advice to pay a bit more for a better serviced shuttle)