Not quite sure what you are asking, a pic would help us. small gaps can be sealed with urethane caulk(not foam). It sticks to everything and never really hardens so it moves with the surface it is on.

 

As per pattached picture the gap between the concrete slabs is 1 1/2" wide and 2-3" deep. I would like to close the gap and make it look nice. The options that come to my mind are: fill with soil and grow low growing grass(this would not be my choice-high maintenance), use wood composite 2" x4" as it was originaly( the gap between the slabs is smaller due to run off from new over coat-would be difficult to squeeze the 2x4 in the gap), use (elastic) concrete and pebbles(concrete aggregate look).
Or better suggestion from any of you with greater experience with such matter.

 

I used some polyurethane-based material that came in a caulking tube. You stuff backer-rod (foam tubing about 1/2 to 3/4" in diameter depending on your gap size) in the gap then lay in this stuff. If flows and then hardens to a flexible joint. HD had the stuff in a color oo three. Might try to find it at a masonary supply for wider color selection or buy on-line somewhere. I forget the brand name sadly.

 

Thanks! After you posted your idea, I googled backer-rod(did not know about it) and ran into another term "Chinking". As I read, realized that another option is to use small pebbles ($4/bag at Home Depot) as a feeler(in place of backer-rod) and top it off with 1010 Chinking. Does this sound reasonable?