I don't know if you wish to stay in hotel or apartments but I can recommend the Sol Principe in Torremolinos there are also the Principito apartments attached to the hotel. The main picture in the url that follows shows the three buidings of the hotel in center frame with the apartments at upper left hand corner. You can zoom in and out to show the surrounding area which is flat and has easy walks along the sea front. Dont be put of by the size of the complex it was not noisy and caters for all types of holidaymakers including the disabled. Great hotel can't vouch for apartments but never heard of any complaints regarding them in the years we stayed there.

cleck here for hotel

I'd nominate La Cala, which is pretty much hill-less near the front. It is no longer the sleepy fishing village it used to be 20 years ago, but still retains a lot of Spanish character.
Think about staying in the Gran Hotel, bang on the beach.

about the flattest place and resonably quiet i know is Torr Del Mar at the far south west of the area known as Torrex costa

I suggest La Carihuela, which is between Torremolinos and Benalmadena and the flat promenade just goes on for miles and miles, starting from the marina in Benalmadena ...all the way passed Torremolinos.

la carihuela is a pretty laid back resort hanging on to its Spanish roots and world famous for its fish restaurants, but there are other resaurants of different cuisine.

There are some video's of the sol principito/principle and of La carihuela that I have made, in this forum.
The video of La carihuela will show you the promenade and there are plenty of good hotels situated on the first line/promenade.

sanjiiiiiiiiiiiiiiii

Just booked today for La Carihuela - Oct 08, love it.