At our school, the room moms are usually picked in late September or early October, so we usually introduce ourselves in the Halloween party letter. We include our email address and phone number for people to contact us with ideas or to let us know if they would like to help with a party. Our school actually sends home a form for parents to sign up to be a room parent. On the form, the parents also have the option to mark if they would like to run one party, help with any of the parties, or just provide supplies for the party. And we always ask for help at each of the parties when it gets closer to the date of the party.
Before you start planning food for your parties, you may want to find out if there are any allergies in your daughter's class and the school's and/or teacher's policy on dealing with any potential allergies.
Here is a web site with a lot of ideas for projects and games for parties: http://familyfun.go.com/
Here is a link to their Halloween ideas: http://familyfun.go.com/halloween/
A couple of ideas that I have used with the younger kids are:
Ghost Wind socks: http://jas.familyfun.go.com/arts-and-crafts?page=CraftDis... It's a simple craft that the kids can do with minimal help.
Horr-edible hands: http://jas.familyfun.go.com/arts-and-crafts?page=CraftDis... This is candy corn and popcorn in clear plastic gloves that can also double as a snack. You can even add a spider ring to one of the fingers for decoration.
If you want to have a healthy snack, my daughter's second grade class had fun making skeletons out of vegetables. http://www.kraftfoods.com/kf/recipes/skeleton-brain-dip-6... We divided the kids into groups and gave them each some hand sanitizer. Then each group got a tray, some dip, a bowl of vegetables, and a picture for an example. Each group made a skeleton with the vegetables. Then after we took a picture of each group with their skeleton, they ate the vegetables. And the kids actually ate most of the vegetables up.
I usually try any projects with my kids to see how it goes before I use it in a party. My kids are pretty crafty, so if they struggle with a project, I know ahead of time that I will likely have 25 kids struggling at the party. If there will be a lot of helpers, that might not be a problem. However, if there are only 1 or 2 helpers, you don't want the projects to be too difficult.
For teacher appreciation day, you may want to keep in mind that it's towards the end of the school year. At my kids' school, we usually try to do a big end of the year gift, so we try to keep the teacher appreciation day gifts small but thoughtful.
In first grade, we worked with the class to make a mosaic flowerpot. http://jas.familyfun.go.com/arts-and-crafts?page=CraftDis... The other room mom and I provided a large pot and the grout. We then asked the kids to each bring in 2 or 3 small trinkets that they would like to add to the pot. We worked with the 2 kids at a time in the hallway and attached the trinkets to the pot. We then added a plant.
In second grade, we brought in a basket and asked each kid to bring in a piece of fruit to put in the basket so the class made a really nice fruit basket for the teacher.
We also have done the vase with each kid bringing in a flower and a jar for the kids and parents to put a note for the teacher.