Short answer: the Royal Botanic Gardens, the St Kilda foreshore at sunset, and the Yarra River walking corridor between Federation Square and the Royal Botanic Gardens. “Nicest” is subjective; here are the places that consistently work for international visitors.
The Royal Botanic Gardens
38 hectares of parkland on the south bank of the Yarra, established 1846. Free entry, open 7:30am to sunset. The Ornamental Lake, the Ian Potter Foundation Children’s Garden, the Australian Forest Walk, the Tan running track around the perimeter.
For UK visitors comparing to Kew or the Chelsea Physic Garden — Melbourne’s RBG is the equivalent in scale and quality.
The St Kilda Foreshore at Sunset
The bayside walking and cycling path runs from Port Melbourne all the way to Brighton, around 14 km along the bay. The St Kilda section — between Acland Street and the Esplanade — is the most-photographed sunset in Melbourne. Luna Park’s heritage entrance, the Esplanade Hotel, and the bay views combine into the city’s most-recognisable bayside experience.
The Yarra River Walking Corridor
From Federation Square along the Southbank promenade, past the Crown precinct, the Arts Centre and the National Gallery of Victoria, ending at the Royal Botanic Gardens. 30 minutes walking, the city skyline visible across the river.
The Carlton Gardens and the Royal Exhibition Building
UNESCO World Heritage listed (the building, for its architecture). The Carlton Gardens are formal Victorian-era public parkland — symmetrical, grand-fountain-and-pathways, north of the CBD. The Melbourne Museum sits in the same precinct.
Albert Park and Albert Park Lake
A large urban park 4 km south of the CBD with a 5 km running and cycling track around the lake. Hosts the Australian Grand Prix (mid-March) on the road circuit around the lake. For most of the year, just an excellent open green space with bay views to the south.
The Brighton Bathing Boxes
Dendy Street Beach, Brighton. 82 colourfully-painted timber bathing boxes — Melbourne’s most-recognisable bayside landmark. Free to walk past; the boxes are privately owned.
The Macedon Ranges and Hanging Rock
50 km north-west of the CBD. Hanging Rock is a 718-metre volcanic outcrop, the location of Picnic at Hanging Rock (1975 film). The Macedon village and the surrounding wineries are a half-day drive trip.
What This Means for You
For a single half-day “nicest place” experience, walk the Yarra River corridor from Federation Square to the Royal Botanic Gardens, then continue south to the Shrine of Remembrance. That sequence covers the city’s three most-recognisable open spaces in 2 hours.
For more, see unique things to do in Melbourne and coolest place in Melbourne.