Lagoon-side, the safe bets are Mont Choisy (kilometres of shade under the filaos, free-for-all tables and kiosks, first come first served), Bras d'Eau on the east coast — a little-known national park where picnic tables hide among the eucalyptus fifty metres from a confidential beach — and Pointe aux Canonniers for its lawns facing the reef. On Sundays everything is taken by 8 am; on weekdays you'll be spoilt for choice.
At altitude, two marvels: the equipped areas of the Black River Gorges, notably at the Alexandra Falls lookout (600 metres up, plunging views over the wild South, guaranteed coolness — perfect when the coast is suffocating in summer), and the Pétrin lawn, trailhead of the hiking network, with its tables at the edge of the native forest. Bonus: monkeys sometimes come to inspect the leftovers — never feed them, they help themselves perfectly well.
The perfect Mauritian basket is assembled at the market that very morning: crusty homemade bread, vegetable pickles, boucané or fried chicken from the Chinese shop, lychees or pineapple depending on season, gato pima for the aperitif and iced vanilla tea in a thermos. Expect Rs 300 – 400 for two, feast included. Supermarkets sell decent cool boxes for Rs 400 – 600 if you don't have yours — an investment repaid in two outings.
Three rules of the local picnic to finish: you take ALL your rubbish with you (beach bins overflow on Sunday evenings), fires only in the fixed barbecues of equipped areas, and you share — offering a lychee to your kiosk neighbour is the island's best conversation starter. Local tip: public beach kiosks can't officially be reserved... but a mat spread out at 6:30 am is as binding as a notarised deed.