Get different coloured crepe ribbons. You can make paper chains with different colours and hang on the wall/chairs. You could tape the paper ribbons to the top of the door where the kids come in/out of the house or into the main room for the party.
You can get multi coloured bangles/hair ties etc for party favours.
Finger painting
decorating paper plates they can take home
multi coloured napkins/plates/cups etc
find the pot of gold/treasure hunt
get different coloured My Little Ponies and turn them into unicorns. Place on table and throughout house/garden
If you have a grass area, create a path with sparkles. Though they'll be stamped back into the house and you'll be finding them in crevices 5 years from now
place multi coloured paper or plastic windmills throughout the garden.
you can do anything really, just use lots of colour.
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