The south coast is a strong destination for couples. The beaches are beautiful, there are genuinely good restaurants at most price points, and the pace of life here makes it easy to slow down together rather than ticking off an itinerary. The towns vary considerably in character though, and where you stay has a significant impact on the atmosphere of the trip.
The best base for couples is Weligama for a shorter trip and Hiriketiya for a longer or slower one. Weligama gives you everything in one place: a long calm bay, good food and coffee, easy access to Galle and Mirissa as day trips, and enough variety that you do not run out of things to do or eat. Hiriketiya is more deliberately slow. The bay is small, the surf is consistent, the cafes are excellent, and there is nothing demanding your attention. If your ideal couple's trip involves coffee and books in the morning, swimming in the afternoon, and dinner by 8pm, Hiriketiya is the right place.
Galle Fort is one of the most romantic settings on the south coast for an evening or two. The fort's narrow lanes, the old Dutch architecture, the sea visible from the walls, and the concentration of genuinely good restaurants in a small area make it a natural destination for couples who like history and good food alongside the beach. Stay inside the fort rather than outside it for the full effect.
Tangalle is worth considering if you are after something remote. The town itself is modest, but the beaches east of it — long sweeping arcs of wild sand backed by palms — feel like a different coast entirely. The sea here is generally too rough for swimming but not for walking, and the atmosphere is quieter and more untouched than anywhere between Galle and Mirissa. A couple of nights in a small guesthouse or boutique property on the Tangalle bay is a good end to a longer south coast trip.
What to do together. Whale watching from Mirissa between November and April is one of the more memorable shared experiences on the coast. Book a boat with a small passenger limit. A Koggala Lake boat safari is an easy half-day that is surprisingly charming and much less crowded than the main tourist beaches. A cooking class followed by a long lunch is a consistently good day. A sunrise walk on the Galle Fort ramparts before the tour buses arrive costs nothing and is genuinely beautiful.
What to skip as a couple. Hikkaduwa. It is a party beach with nightlife that caters to a backpacker crowd and has little to offer couples who are not looking for that atmosphere. The main Mirissa strip after dark is similar. Neither is worth a night of your trip when Ahangama, Weligama, and Hiriketiya are nearby.
One practical note. The south coast does not have many formal fine-dining restaurants the way a European or Southeast Asian beach destination might. What it has is small, excellent, owner-run restaurants that change their menu based on the day's catch, serve genuinely good food, and feel nothing like a hotel dining room. That is better. The best meals on this coast happen at tables of six with no menu, where the fish was landed that morning.