When my kids were small they loved to get some old blankets and make a fort under the table! They would take snacks under there and play for hours! I am not sure what or who they were pretending to be but they really enjoyed themselves!
For young ladies(and gentlemen) there is nothing as fine as having a fancy tea party! Everyone dresses for the occasion including the dolls, stuffed animals and of course the grown ups. (Browse thrift shops and Goodwill stores for costumes and fancy hats to have on hand for such an occasion!) Help the kids make a little snack to serve with the tea. Some little finger sandwiches are also easy to prepare. Make sure it is a grand fete with all the trimmings. A really great tea party takes a lot of preparation and can keep kids busy for hours. Let them craft and hand print their own "fancy" invitations, prepare the table with the finest table linens and tea sets and help with the food preparation. This will take a little foresight on your part to make sure all these items are handy just in case you suggest a tea party to a group of bored children! The boys might enjoy the role of "Waiter" to the ladies. You might also suggest that he use a proper English accent while seving. The girls are sure to get a giggle out of that!
Plays and skits are also a fun time for budding young thespians. Again it must be a big fancy deal for everyone and taken very seriously. Have a makeshift stage, costumes and props on hand(a dime a dozen at thrift shops or after holiday sales). Have seats arranged for the audience and make sure all guest show up in proper dress for a night of theater! Call the cousins and neighbors if you like. Kids love feeling important to grown ups. There are lots of sources online for plays and skits for every age group. You could also have the kids make their own sock puppets and a makeshift stage of cardboard boxes. Oh, how kids love cardboard boxes! I bought a new stove when my kids were little and the kids played in that box for weeks! For tweens and teens let them browse a cookbook for an ethnic dish that looks appealing and have them prepare it. Ask them to research the ethnicity and culture where the recipe originated and discuss it over dinner. If they choose Japenese for instance, have them serve it on the floor in the traditional manner. Make sure everyone takes them seriously and always make them feel dignified and important. I hope this helps!