what are the memorials erected in that place?
The Cavalry Memorial,  the Diana, Princess of Wales memorial ,Holocaust Memorial , the 7 July Memorial and others.
Hyde Park, London
The Cavalry Memorial was built in 1924 at Stanhope Gate. It moved to the Serpentine Road when Park Lane was widened to traffic in 1961. South of the Serpentine is the Diana, Princess of Wales memorial, an oval stone ring fountain opened on 6 July 2004. To the east of the Serpentine, just beyond the dam, is London's Holocaust Memorial. The 7 July Memorial in the park commemorates the victims of 7 July 2005 London bombings.

when did hyde park open to the public In 1637. who designed this park? The first coherent landscaping was done by Charles Bridgeman. what's the area of this park? 142 hectares. what are some of its features? Popular areas within it include Speakers' Corner, and Rotten Row. what plants will you find there? The weeping beech , roses and others.
wiki:19523258
Hyde Park, London [SEP] Statues and sculptures
Made of Cornish stone, it was originally part of a drinking fountain, though an urban legend was established, claiming it was brought from Stonehenge by Charles I. An assortment of unusual sculptures are scattered around the park, including: "Still Water", a massive horse head lapping up water; "Jelly Baby Family", a family of giant Jelly Babies standing on top of a large black cube; and "Vroom Vroom", which resembles a giant human hand pushing a toy car along the ground. The sculptor Jacob Epstein constructed several works in Hyde Park. His memorial to the author William Henry Hudson, featuring his character "Rima" caused public outrage when it was unveiled in 1925.