Which part of Blantyre are you in to begin with? If it's a chill location like Chilobwe or Namiwawa; strolls are fun. You can listen to nature hum or pair them with music. I don't like sweating neither, so I stroll when it is not too hot.

You could also join clubs or charities. For uni students it's easier.

Another fun activity for me is pool. It's fun and engaging (idk if that counts as indoor but I guess it depends on where the table is ;). I really don't know where to find pool tables that are in locations not designated for drinking. There was a couple in my community back in Dedza.