Provide each student with a card letting them know how much you appreciate them being in your class. Have a snack day with special treats. A movie day with popcorn.
Someone that may be able to help with this is Optimist International. My husband and I have been members for years and our group in Denver honors one student a month, in November (I know I'm a little late here but maybe next year) we have a actual week- Optimist Youth Appreciation Week.
Their web site www.optimist.org
Oh yeah did I mention their theme? Friend of Youth.
If you are having a student appreciation assembly, consider all those individuals in the community that appreciate the children.
Invite the Mayor to come say just a few words while commenting on how important the students are to the success of the city community. Maybe invite the city librarian to come say just a few words on their spin regarding the value of students and how much the librarians learn from students and the books they look at, the ones they place back on the shelves, and the ones they check out. Bet the kids didn't realize how valuable their thoughts and actions are to the librarian and the city library. Maybe if your school has a business partner (like a bank) invite a few more people to say just a few words on their value to those as well.
Having student appreciation events are great. I like to do them throughout the year. Cup cake day is simple with a few bakers per class, and ice cream socials after school. Dances are in order and pizza parties with open gyms/movies. We also provide relief snacks during their testing days.