This is complicated because your sentence contains 2 topics:
- relative clause
- some + thing(s)
First, "that is", which is a subject pronoun + a verb, can be omitted for this case.
There is a dog (which is) happy.
-> There is a dog happy.
->> There is a happy dog.
Secondly, "something" is an article "some" + noun(s), this "some" acts the same as other articles for this case. But when "thing" that follows the article "some" is without an "s", they become "something" instead of "some thing"
There is a dog.
-> There is a good dog.
There are some things. ("some" = article)
-> There are some good things.
There is some thing. ("some" = article)
-> There is something
->> There is some good thing.
->>> There is some thing (which is) good.
->>>> There is something good.