Optimisation is a good topic. Start with something big - I'm guessing Jakarta's traffic flow could be improved - "what's the optimum speed for vehicles to get best traffic flow" (there's a Wolfram Demo Project that simulates this already). Then step back and hit some simpler optimisation, give a piece of card and make a box with the greatest capacity. Got access to some MAB blocks or similar? Use them to test the students' designs. Then do the mathematical modelling and Plot the volume to see where it's at its greatest. Depending on the student level you can get off here or differentiate and Solve exactly. Build up some more difficult problems and then go back to the traffic problem.
It depends on the group but I often find this topic engages students more than other high level maths topics because it's about doing things better, faster, cheaper... and it can be done without calculus or with it.
Here are some problems with models that I've created if you decide to go this way: Optimisation Problems