Age group(s) for this birthday party activity:
Preschool and Younger
Elementary School

Birthday party planning information:
Scooby Doo and Friends were sleuthing clues long before a dog named Blue came on the scene. Invite them to your next party; there are many ways to Get in the Groove.

Scooby and friends require food, of course! Sandwiches, mini-burgers served in buns, hot dogs (dice finely for smallest guests), or other finger-foods work well. "Scooby Snacks" can be placed on the serving table, too (chips, snack crackers, fruits, veggies).

Activities will vary according to age. Dressing in 70's clothes is certainly fun! Face Painting (peace symbols, flowers), perhaps making Tie Dye T-shirts from a kit (wear aprons or old clothes for that).

Love Bead necklaces are fun to make, with red licorice laces, Cheerios, and Fruit Loops.

Have some elements of "mystery" to solve: One way is to have a few Mystery Items in paper bags, preferably a couple o' Layers of bag(s), taped shut with packing tape, and allow the children to take turns shaking and feeling the bag to guess what's inside. Ask them to wait until everone has had a chance to check out the bags, and then shout out their guesses. Then, tell what's Really in the bags. Have several on hand. Older children might enjoy sitting in a circle and playing a Blindfolded version, without the bags, just feeling objects to guess what they are. You can even use elements of Scent and Sound!

Or, try a Treasure-Hunt adaptation.
Show the children pictures of Scooby-Doo type items, and have the Real Items semi-hidden in the general area of the party. Send the kids to find them, and draw pictures of each item they locate. (Shy children can work with a partner). With older children, or with parent partners, the search can be organized slightly more like a Treasure hunt, with one clue leading to another and to another, but keep the Action confined to the main party area. It's nice to have the luxury of a back yard to work with, and sunny weather, but it can work indoors, too.

When you have finished the Hunt, move on to something else, perhaps the food?

Or perhaps Dancing! ...'You Ain't Nothin' But a Hound Dog' (Elvis)...'Green Tambourine'...
Oldies collections of the 60's and 70's are available, on tape or CD, or just tune in to an area Oldies station to set the mood. Jump, twist, shout, boogie, lip-sync, and play Air Guitar. "Wild thing, you make my heart sing, you make everything...groovy..."

Mummy Wrap! Are you the Mummy that's always chasing Shaggy and Scooby? Get out the spare rolls of TP, and let party guests take turn making "mummies" of their friends or parents. Take lots o' pictures/video of all this insanity...

Bean-Bag Toss into a huge Dog Bowl works. Creative types might work up a Bean Bag toss featuring an enlarged Scooby drawing with a hole cut for the Mouth, as the obvious Scooby Snack Beanbag Target, but even a coffee can catches bean bags, with a satisfying "thunk" noise.

Deluxe options for Party favors would include items like a Scooby Doo book, or a plain notebook for recording Clues, or a plastic magnifying glass. A Scooby Doo sticker on ordinary items can "coordinate" them with any special Scooby partyware you'd care to use.

It's not a party without Cake! If you can find small age-appropriate plastic Play Sets with Scooby and Shaggy and the whole gang, they can be washed lightly and dried, and used as cake toppers, or you can utilize cake toppers sized and sold for that specific purpose.
Around our house, Colored Sprinkles are always the classic, easy, yet elegant touch for a home-baked, frosted cake. Scalloped Borders are cool, if one has the time and talent, but Sprinkles absolutely rock, always. ;-)

If you'd care to make a Mystery Machine cake, resembling the van Scooby and friends travelled in, you will need a Loaf-shaped cake. It helps to consult a picture of the Mystery Machine, as on the plates or napkins, or make a sketch next time you watch TV, but don't get too crazed about tiny details!

The Mystery Machine is sort of an aqua blue (turquoise is too deep to be appetizing...)with some light green (vs. electric lime) race stripes/streaks along the lower part of the body. A one-pound tub of frosting (white, divided and tinted) will be sufficient.

Oreo Cookies make great wheels for the van cake. You can use green decorating gel for the license plate, with or without a oval or rectangular cookie for the plate. Windows can be piped on with black icing gel, chocolate brown icing, or outlined with black string licorice. (or try cutting some fruit leather shapes, for "tinted" windows). There is probably no one right way to make a Mystery Machine, and the kids will Love It!

Add some Flower Power with sugar candy flowers, flowers piped on, flowers made with candies like Smarties or mini M&M's, or whatever you can come up with that looks....Groovy. A cake topper Scooby can sit on top of the Mystery Machine, if you like.

Other suggestions or comments:
If you use the really Excellent Icing Colors from the cake decorating specialists (vs. the grocery store box of 4), some color change will happen over time. The icing should sit just a bit, until it has Intensified all it's going to; then you can decide if you need to add more. It really doesn't take much; those colors are highly concentrated, and it's always Way easier to add a li'l more than to Lighten something by adding more frosting.

Sandwiches can be cut into Dog Bone shapes with metal cookie cutters, if you wish! Stick to basics like PB&J, or ham and cheese. A butter-type spread, used thinly, will work with most kids, but some intensely dislike mayonnaise...it can always be at table as an add-on.

Foods with choke potential, such as hot dogs, popcorn, grapes, fruit chunks, should be placed Just out of Reach of the smallest guests, so that parents can assist them in selection. Perhaps some Small Dice Finger Foods of the same menu items can be readily available, or encourage parents to come into the kitchen and dice and chop! Safety is key to any successful gathering.

Yet one last "Scooby Snack" idea that could be a parting gift: a small container of home-made chocolate chip cookies, with a bright Bow on top, and a "Scooby Snacks" label on it. The container does not have to be fancy; if you save up some cleaned-out Frosting Tubs and Lids, those are about the perfect size and shape! Scooby-Doo stickers or paw prints look real good on that.

Frosting Tubs are also a perfect storage container for homemade Play Dough, but that, of course, is labelled Do Not Eat. (I think the Salt would be a deterrent... ;-)