What You Did
You made a homemade lava lamp. You observed fun chemical interactions from the materials you mixed together.
You should have seen bubbles that moved up and down
If you didn't leave enough room at the top of your bottle or if you added too much Alka Seltzer at one time, it might have bubbled over
Vary the temperature of the water. Vary the proportion of oil and water. Vary the amount of Alka Seltzer you add to the bottle. Vary the size of the bottle
pH, base and acid reactions, effervescence