Welcome to the site and to your new job, Tabby! It takes a special person to ensure that these old dears have fun on a daily basis (sorry, assuming you're working in long-term care).
I've never worked activities myself but do know that the job is much harder than it looks. What I can tell you is try to be patient with the nursing staff; I've had more than one occasion where I'd have to pull a resident from an activity, not so much because it's a convenient time for me, but because they require a procedure that has been ordered for a particular time. I know this annoys the activities people, but sometimes it can't be helped. Personally, I always make a point to apologize for the interruption and return the resident to his/her fun as soon a possible, but you may find some nurses or CNAs will just come in and take them without so much as a "by your leave." These people aren't trying to be mean, they're just trying to get their jobs done in a stressful work environment.
You can also utilize the staff; when you're new to a place, you don't know each individual's habits, likes/dislikes, diet restrictions, etc. and it can be pretty overwhelming. If you have a stable staff, your nurses and CNAs are a great resource.
Sorry I can't be more specific as far as particular activities...I did find a pretty neat website that lists some ideas:
Good luck to you; you are in such a rewarding field.