As long as you can fit it into the bottle and you make sure the stopper is leak proof then your imagination can run wild. Please make sure any liquids are non toxic in case of any spillages or accidents. Or stupid people trying to drink contents, humans are a bit stupid and will try anything.
If you are going to use a liquid the stopper should be either rubber or plastic or a screw cap, all should still be glued on. Be aware if you knock your bottle over just after gluing stopper then the contents WILL get stuck to the inside of said stopper. This has happened to me a few times and I have had to sell the item at a discounted price because of this slight flaw.
Other than that have fun with your imagination and create something fun and beautiful.