ni suru — ことにする

The meaning of this construction is analytic. 

x koto ni suru 
→ to decide on x thing

ikanai koto ni suru.
→ I’ve decided to not go.

It's hard to see how 'ni suru' can mean 'to decide on' until you remember that 'suru' also means 'to make'.

x koto ni suru 
→ to x thing, I make it so

So now the meaning of 'to decide' falls out naturally.

I decide on x → I make x so

ikanai koto ni suru 
→ I make 'not going' thing so → I’ve decided to not go

koto ga aru — ことがある

The meaning of this construction is analytic. The meaning is simply, “there is such a thing” as the clause.

Nihon ni itta koto ga aru
→ The thing of having gone to Japan exists.
→ I have been to Japan.

Okorareru koto ga aru
→ The thing of getting scolded exists

→ Sometimes I get scolded

Asanebō suru koto ga aru

→ The oversleeping thing exists

→ Oversleeping is a thing that happens.

→ There are times when I oversleep.

Sushi o tsukutta koto ga arimasu ka? 

→ Does the having made sushi thing exist?

Kono eiga wa mita koto ga aru to omou 

→ I think the having watched this movie thing exists

'Sometimes x' and 'there are times when x' is the closest equivalent in English.