Hey, welcome to the board! Nice to see new faces around here always.

With events you can create content in your level like what you mentioned. You can use MSB3000's method to add an event, or right-click a tile (in layer 4) and select "Select Event" in the menu that comes up.

Lists of events and event descriptions might help you find the events you want to use: Event List on Howto JCS - Event Categories and Event Descriptions at J2O's encyclopedia.

You'll find coins in the Pickups -> Goodies category in JCS, and enemies in the Enemies - > Bad Guys category in JCS events. If you want to make the coins come back after a while, use a generator.

Hope this helps you.