All towns and cities have markets and the Sigatoka markets are worth a visit - in fact one of the best markets in Fiji. The sigatoka region is well known for its export quality fruits and vegetables that you find in supermarkets overseas. Buy straight from the source and choose from a range of fresh fruits and vegetables, especially recommended buying some of the cheap and plentiful in season fruits, quality which you wont get at most other markets. You can have a bowl of chillies or a heap of pineapples. 

If you're looking for photo opportunities the coil of tobacco is magnificent as are the carefully wrapped kava roots. 

If you're a traveller, of course, you won't be buying produce - but a few little trinket stalls well worth a visit.  You could buy some groovy floor mats made by the local ladies - recycled fabrics on rice bag backing - great to take home, wash in the machine and supporting local industry.  These mats are beautiful - best present to bring back from Fiji - all sorts of colour ranges. 

Buy some bananas to enjoy in a fresh bread roll from the nearby Hot Bread Shop, or a beautiful fresh-cut pineapple.  There's usually a local lady selling nice Fijian home baked cakes - be adventurous, support the local farmers and EAT!