Cleveland, the country house of the Palmers is a commodious place, modern-built and situated on sloping green lawns.
Apparently, in Austen, a 'modern' house usally means built no later than mid-Eighteenth century.

The grounds are described;

'It had no park but the pleasure grounds were tolerably extensive;...it had it's open shrubbery, and closer wood-walk; a road of smooth gravel winding around the plantation, led to the front, the lawn was dotted over with timber; the house itself was under the guardianship of fir, the mountain-ash, and the acacia, and a thick sceen of them all together, interspersed with tall lomardy populars shut out all offices.'
(ch.42)
For Marieanne, the house is not evocative, yet places closest to her heart seem relatively short distances. 'Marieanne entered the house with a heart swelling with emotion from the consciousness of being only eighty miles from Barton, and not thirty from Combe Magna.'
Cleveland's grounds appeal very much to Marieanne's sensibility-
they offer her the last occasion in which she indulges her exaggerated emotions,
'In such moments of precious, invaluble misery she rejoiced in tears of agony to be at Cleveland'.

The grounds appear to have touches of the picturesque.
JA describes them as 'like every other place of the same degree of importance'- that is, satisfying contemporary taste for pleasure grounds.
Cleveland is a new house- My impression is the presumably newly laid out pleasure grounds were in the style of Repton (mentioned in MP), the last great C18th landscape gardner. In the 1790s', Repton used aspects of the picturesque landscape gardening.
Similar to Capability Brown who has been mentioned this GR, Repton's grounds remained green and natural with clumps of trees. Partly in response to clients influenced by the picturesque; Repton introduced more features such as walks, the conservatory...Cleveland has winding shrubberies with dry gravel walks and a green house. Colour was revived with flowers.
Lane comments JA was in sympathy with many of Repton's ideas.

What appeals most to Marieanne is the temple, 'stealing away through the winding shrubberies..., to gain a distant eminence; where, from it's Grecian temple, her eye, wandering..., could fondly rest on the further ridges of the horizon, and fancy that from their summits Combe Magna might be seen.'

Apparently, temples were not part of the picturesque movement which I believe prefered such landscape features as waterfalls, caves and grottoes. However, Brown sometimes included them in his landscape designs when customers wanted a temple. I don't know if it indicates Mr Palmer had a Classical education or that he did the Grand Tour.

Or maybe it's just an ornament in the Palmers' garden.
The English Greek Revival of JA's time probably gave many people the fancy for a Greek Temple in their grounds.

By a temple JA may suggest Marieanne thinks of herself as a heroically tragic. A temple may also signify hope in the way a ruined folly cannot do; Unlike Eliza, Marieanne is not a ruined girl.
Marieanne indulges in solitary rambles all over the grounds. Possibly she is at her most Cowperesque if she quietly recites poetry. She rambles to the most distant parts, 'where there was something more of a wildness than the rest, where the trees were the oldest, and the grass was the longest and wettest,...-given Marieanne a cold so violent'.
A 'wildness' on the edge of the ground was a picturesque/ Repton touch.
Marieanne's longing for Willoughby is associated with her lapse of health. Emotionally as well as spiritually, Marieanne appears near breakdown. Cleveland is merely a place of desire, of hopeless longing. She is torn asunder by her heart; between her old ideals yet seeking isolation and solitude.

Within the house, the sickroom is a place of healing.
Marieanne tells Elinor, 'My illness has made me think-It has given me leisure for and calmness for serious reflection. Long before I was enough recovered to talk, I was perfectly able to reflect. I considered the past; I saw my behaviour since the beginning of our acquitance with him last autumn, nothing but a series of imprudence towards myself, and want of kindness to others.' (ch. 46)

Marieanne experiences a moral awakening from her
self-centred love and grief. She acknowleges her excesses of sensibility.

Below there is a link explaining more on landscape gardeners,
Brown and Repton.
A further link will also take you onto a page on Humphry Repton with some watercolours of his work.

References:

http://www.gardenvisit.com/Brown.

M. Lane 'Jane Austen's World' (Carlton Books. 1996)
pp102-104.
M.Lane. 'Jane Austen's England'
(Robert Hale. Ltd. Reprinted 1995)
pp.24-25, p. 43, pp.115-117, pp.185-186.

Mavis Batey. 'Jane Austen and the English Landscape'.
(Barn Elms Publishing. 1996).
Chapter 8.

R. Gill and S. Gregory.
'Mastering the novels of Jane Austen'.
(Palgrave McMillian. 2003).
p.111.