For decorations, to keep it simple, put up red and white streamers. Talk to the manager of your local McDonald's and they may let you purchase, or even give them to you, some of the "Happy Meal" bags/boxes and you can serve lunch in those. You may want to call a week or more in advance to order them, depending on how many you need.

For activities. You can purchase Face Painting "crayons", that you just have to dip lightly in water, then you can paint there faces to look like Ronald or the Grimace, Hamburgler, The Fry Guys etc. They come off easily with soap and water and DO NOT STAIN. You could see if you could rent a Ronald McDonald costume, or other characters, and have someone dress up in them and make balloon animals and do the face painting.

Make Ronald's face on the top of the cake and place this in the center of the table as the center piece.

Games
Make Ronald's face on a cardboard box and cut out the "mouth" and have a bean bag toss. Play pin the top bun on the Hamburger.
Play Ronald McDonald Bingo: Make your own Bingo game - See if you can find stickers or draw the faces of the McDonald's Characters plus, hamburgers, French fries, twist ice cream cone, Chicken nuggets etc. onto bingo game boards. Make duplicate pieces to "draw" out of a hat and play bingo.

Even if you do not use any of these ideas I hope it will at least get your creative juices flowing. Let us know how the party turns out and/or what you decide to do.