I am thinking of going on a trip solo to South Korea this dec, anyone had any recommendation for areas location to visit? Any theme park worth visit other than Lotte World and Everland as well?

It will be bitterly cold in December with possible light snow. Plan accordingly. The southern part of Korea (Busan / Jeju) is more likely to have decent weather & no snow. The KTX bullet train from Seoul to Busan is around 3 hours.

Seoul is worth a few days, then yes the south is pretty. Everland and Lottoworld are not parks....

You'll find loads to do in Seoul - the popular places for walking around and taking some coffee, like Samcheongdong, Insadong etc. can be really lovely on cold snowy days :)
Ice skating is good, if you meet someone to go with you or don't mind going alone - there's a rink at Lotte World (also part of Lotte World is indoors so it's good to go there in winter!). But I think outdoor rinks at night are best, for the atmos.
Lastly check the Visit Korea site for winter festivals. This is one I went to last winter, but it was in January:
http://londonseoul.blogspot.co.uk/2012/01/hwacheon-ice-fishing-festival.html
There should be those kinds of ice festivals in december too!

Any affordable lodging have in mind? I remember my friend stayed in guest house or some sort the last time she went, can't remember the name but i think it was quite cheap

Jeonju is worth a couple days. 8-10 days is far too short. If possible you should plan a longer trip.

If possible I also wish to plan a longer trip! But that is subject to my leave and my work. Hehe..