Sure, here are the sentences filled in with suitable relative pronouns or adverbs:

11. That was the time **when** we all felt happy.
12. He paid me 5 dollars for cleaning 10 windows, most of **which** hadn't been cleaned for ages.
13. There are a few things **that** we can do during our summer holiday.
14. The woman prayed all night, **which** kept us awake.
15. That girl, **whom** we talked about, was a famous singer.
16. Many things **that** were given away can be reused.
17. Is this the book **that** you asked me for?
18. I don't find the person **whom** I'm sharing a flat with a very friendly flat-mate.
19. Here is the address **where** you should write to.
20. I can assure you that David is a man **whom** you can absolutely depend on.