I have completely forgot but which items can actually go in the fire for fireside? I want to buy a few extras to thrown in to kick it off when fireside starts.

It's usually the items (beanbags and plushies and stickers in particular) found floating on the site during that time, and the items found in previous years. Like these, from past years (the items will probably be different this year, but they'll probably be plushies and beanbags and stickers):

Subetalodge will have a more comprehensive list. Interestingly, a lot of the items the Rreign gifts you won't be accepted back, so it's not as big a list as one might hope.

It won't take this back, for example:

The wreathed items you get in return, if you give them back to the fire, will get you scorched instead of gifted:

Thank you very much. I think I was on the right track then :)