u mean 1 day only?

on sunday most probably the place like Nami Island & Everland
would be crowded. Lotte world is nearer since Nami Island & everland
have 1 to 1.5  hour journey.

But as for me who just been there last week...i wud recommend u to go
to Everland or Nami Island.
U have to get the earlier bus since one day wudn't be enof...hehe

for Everland - the easiest way is to take a subway from where u depart to
Gangnam station. (Line 2)
Subway + Bus                                                                                                                                                                                                                                               
At Exit 6 of Gangnam Subway Station (Seoul Subway Line 2),
take City Express Bus (jwaseok bus) No. 5002 (50 min ride).
At the Exit 6, walk straight about 50 mtr until u find Giordano, u
will see a bus stop...there will be and ahjussi who incharged, he will
surely help u...

if u choose Nami Island, u have to book a package. the package cost is
23000w (est. RM65). The package includes bus & ferry. U must take the bus
at the Tapgol park / Pagoda park (same place just different name) at Insadong area.
U may stop at the Euljiro 3 (sam) ga sation and walk at exit 5 (insadong) or stop at Angguk station.
be there before 9 because bus will depart at 9.30. For safety, please book first.

and after u get back from nami island maybe u can go ahead to N seoul Tower at Myeongdong...
It is a must...variety of goodies & very nice place