Ok ....
Here is a question to ponder. If you take a particular plane ... lets say a "warbird type" plane that usually doesn not have the best floating charactisitics when landing ... and you wish to improve that aspect ... what is the best approach that does not include flaps. Thicker airfoil? Longer span? More Dihetheral?

Of the three choices you give, increasing the span (which also increases the area if the chord remains the same) will give a little better glide. The increased thickness may have a little effect. Increasing dihedral will have no significant effect on the glide.

You didn’t mention the most obvious and by far the most effective change, which is to reduce the weight. A light wing loading will be quite effective in producing a slower glide and a better sink rate.

Lower drag produces a flatter glide angle while lighter wing loading will give a lower sink rate.

LouW is right - a more cambered airfoil would also help - at the expense of inverted performance.

In terms of "floating" close or over the runway - e.g. a half-span away in altitude, ground effect will affect glide angle and efficiency. Ground -effect is proportional (in some way) to aspect ratio. It works by dampening wing tip vortices producing lower induced drag - or a higher effective aspect ratio. For a given wing area, a wing with a greater aspect ratio will be more subject to ground effect at a higher altitude than a wing a lower aspect ratio - because its span is greater for the same wing area.

In terms of "floating" as a term for getting back to the runway deadstick, reducing weight does not necessarily improve glide - in terms of "glide angle". For example slope soaring sailplanes often use ballast to increase speed and reduce glide angle at the expense of sink rate. But ballast has to be added with recourse to an understanding of that particular aircraft's utility and design. Sometimes the best lift/drag ratio is to be found high up in the speed range, and with other airfoils best lift/drag ratio is at low speed (for a given aircraft at a given weight). Some airfoils are designed to have a wide range of low drag co-efficients (what is termed a drag-bucket) other airfoils even have double-drag buckets (more unusual). Certainly best sink rate on an airfoil is often found at a relatively high lift co-efficient - i.e. some what close to the stall whereas best glide angle will be found at much higher speeds.

With a war bird the chances are the wing will already be highly loaded though and reducing weight (rather than adding it!) will be the best way of achieving "floating" characteristics.