Go for a stroll on the beach. There is a high chance that the South Easter will be blowing tomorrow so Clifton is a safe bet and a great beach none-the-less.
Head off for a unset picnic with a view. Signall Hill & Chapmans Peak offer unrivalled views and you could pre-order some food that both of you like and pick it up on the way. Pizza or finger foods from woolies are always a great option.
Table Mountain sunset special R97.50 pps, more info over here
Putt Putt in Sea Point, for a tiny fee this offers great fun and with the sun setting will also have romantic appeal.
Walk through Green Point Gardens. There is no entrance fee and there are ample picnic tables & grass to relax on.