"The bear had decided to drink the beer."
Translation:De beer had besloten het bier op te drinken.
Yes, it works :) There is a little nuance however:
'De beer had besloten het bier op te drinken' suggests the bear decided to drink all the beer
'De beer had besloten om het bier te drinken' only suggests that the bear decided to drink the beer, though it doesn't state whether or not he wanted to drink all of it
I found this: http://context.reverso.net/translation/dutch-english/op+drinken. There seem to be many ways to translate, such as "...drink to that", so your guess appears to be valid.