Something to consider for simple fun are LED Throwies. This involves very inexpensive batteries (CR2032), a magnet, and an LED. Total cost, under 25 cents total. Search for “bulk LED” on eBay and you will see many suppliers in Thailand and China. Under a dollar with free shipping for 15 to 20 LEDs is quite common. Run similar searches for bulk CR2032 batteries and you will see them for similar prices. Then run other searches for bulk rare earth magnets and you’ve completed your shopping list.
Here is a link for quick demonstration on what to do.
One of the neatest ideas I have seen is to insert an LED and CR2032 (no magnet) inside balloons. Fill the balloons with helium. Attach the balloons with string to each other. You end up with an arch or string of balloons that are lit up in different colors. Fantastic at night and in the dark. Great for parties.
Depending on the quality of the battery and type of LED, I have seen them stay lit for up to 3 days. Enjoy!