You can still plant silverbeet. It needs soil enriched with a good feed to grow quickly. Water regularly and feed with a high nitrogen fertiliser fortnightly (urine?! ideal).
Sweet potatoes are ideal for hot wet summer months. You can grow them from 30cm tip cuttings or from the tops of sweet potatoes. They are ready to dig up when the leaves start to go yellow about 5-6 months.
Sweetcorn needs a lot of nitrogen. Dig the soil well before planting. Keep up the nitrogen fertiliser (it is essentially a grass). Ready to harvest when you peel back some leaves on cobs and press a fingernail into a kernel and it is milky. Eat promptly. Corn earworm is a pest of sweetcorn – treated by dusting the silks on female flowers with an insecticide.