Suggestion 1. Design a built-up truss.
Use the Pratt Truss as template. It is one of the simplest to design and make. It has horizontal top and bottom chords which you can use to support a suspended slab.
Suggestion 2. Wide-flange beam.
Wide Flange Beams can be designed with large spans and are easy to install.
Suggestion 3. Design a flat slab RCC floor.
A flat Slab combines a Beam and Slab
Suggestion 4. Use Pre-stressed Beam
Beams with tensioning tendons allow for long spans without increasing depth. This is usally employed in multi-storey RC Buildings with clear spans 10m or larger.
Final Verdict: If you value speed and ease of installation, I would strongly suggest Suggestion 2. Suggestion 1 has the best strength to weight ratio, followed by Suggestion 4. I would not normally recommend Suggestion 3 unless the architect wants a clean and flat slab soffit.