Rideshare is 100% the way to go unless you are one who is particular about the type of vehicle that's driving you or need personalized red carpet service. (Nothing against those who do)

You can test the pricing on grab by selecting the appropriate region (Indonesia) and then inputting the two endpoints. I'd expect either grab or gojek to be a fraction of the cost of any professional service and very surprisingly cheap for an American.

For sightseeing, really depends on what you're wanting to see. Chances are good that your rideshare driver from the airport will also offer to be your sightseeing driver and you can negotiate something directly.