Build a cmu block wall on two sides, face it with stone and do a concrete counter top acid stained and sealed and cantilevered 12" so you can pull bar stools up to it

You can't really "cover" the gas heater - but you could try to "disguise" it using a 'low growing' bamboo or informal hedging plant in a container that you can keep pruned and - that way you can move the plant if you need to access the heater for maintenance. If you were to put a storage unit under it you could easily disguise the heater with rosemary, or grass plants etc. Be sure to check what load bearing capacity your balcony has - so that you can safely use your balcony to its full advantage. Mirrors make a difference on the opposite wall to where your heater is - you could even use a one on that wall too - if you use a mirror opposite a garden wall your space will instantly feel bigger. To keep the maximum floor space you can on the balcony, think about a vertical wall garden and planter boxes on the railings. If you do decide to go for planters on the floor of your balcony remember to go light weight plastic as terracotta, and porcelain can get heavy quickly. For your soil - think about core peat with a slow release fertilizer - core peat is light weight but has no nutrients for your plants. I cant see that you've got a tap - if you don't then think carefully about drought tolerant plants as carrying out watering pots several times a day in summer isn't much fun. If you do have a tap - think about installing a drip irrigation system on a timer to give yourself more time to sit back and enjoy your balcony rather than working on it.

Perhaps you could train something on a frame on the exterior of the retreat and encourage it to spill over and inward (to save on space within the room -obviously not for instant results!) some potted screening plants like clumping bamboo will give more instant impact, but it doesn't hurt to plan ahead. I would like a garden bench with somewhere to put your feet up and rest a drink at the rear of the retreat surrounded by potted plants (perhaps a fragrant vertical herb garden on one wall - I imagine it would need to be a freestanding structure) Is there space in the covered alcove to place a round patio table and chairs and still move through he door way? Outdoor furniture survives better under cover but if small and not too heavy it could be moved out on sunny days

Yeah I reckon be super careful with this one....be conscious of making changes that will impact the character and balance to the facade.......when it comes to this era and style, scaling of the fenestration is really important......below is a project that we developed a new home, but the client wanted to reference the homestead type of building of similar era to yours.....that entry door with the highlight window is an important thing to keep IMO, possibly you may need to get some further architectural assistance to ensure you don't make changes that are incongruent to your beautiful property