A one-day suggestion
Walk through Ibla in the first part of the day, then head to Marina di Ragusa. Spend the afternoon on the beach or promenade, depending on the season and sea conditions.
If you have two days
Separate the two experiences: one day in the city and one on the coast. You will have more time for breaks, visits and changes of plan.
Before deciding
To combine both in one day, consider transport, breaks and the return journey. Without a car, check current connections before setting your itinerary.