There are lots of possible options. This is not an exhaustive list, but it covers some of the main options:
Solid State Thermoelectric generators
For lower temperatures (i.e. below the melting point of solid materials), a Thermoelectric generator is your friend. The Seebeck effect is when a voltage generates over a length of material that has a temperature gradient (i.e. heat flux) across it. Currently, this has some use in small, low power devices (tens to low Kw). With future handwaved technology, it may be possible to create exotic materials with a much stronger Seebeck coefficient, hence enabling more efficient power conversion.
This is used extensively today, but it fits the bill of not requiring boiling water. Some sort of gas is heated, expands, drives a turbine, cools, and is run through a compressor. This thermodynamic cycle is essentially a closed cycle jet engine. This is used for modern gas turbine power stations (in conjunction with a more conventional steam based cycle), but it is an efficient use of high temperature sources that work at many hundreds of degrees.
If your thermal source is hot enough to (at least partially) ionise a gas into plasma, then you can use a MHD generator. The hot plasma is ducted down a pipe with a transverse magnetic field. This separates the positive ions and electrons, into two streams, extracting work from the gas/plasma and slowing it down. The separate streams can be passed over high temperature electrodes, which form the terminals of a circuit, creating a voltage difference.
If your thermal source is extremely high temperature, and directly produces a stream of charged particles (perhaps some exotic fusion reaction) you may be able to directly convert your exhaust. There are a lot of different ways this can be engineered from schemes that look similar to a MHD generator, all the way up to devices that look like a particle accelerator that operates in reverse.
If the high temperature source is extraordinarily hot, it may radiate a significant quantity of blackbody radiation. Enclosing it in some sort of exotic photovoltaics could generate electricity. The "exotic" part comes in because current photovoltaics are only really suitable to harvest sunlight. You may need some exotic (i.e. handwaved) semiconductors to make them work at higher frequencies (perhaps even x-rays or gamma rays), and with efficiencies that make it worthwhile for your alternate heat source.
All of the above
You may notice that there is a bit of a hierarchy to the above systems. As temperature increases, the most efficient engineering solution changes. For even more efficiency, you can chain them together! The most efficient system may be one where, for example, the exhaust of a futuristic aneutronic fusion reactor feeds into a direct conversion generator, which exhausts a lower temperature plasma into a MHD generator, which is cooled by a molten salt loop/heat exchanger into a Brayton cycle gas turbine, which is finally topped off with a "low" temperature coolant loop through a bank of thermoelectric generators. All of this would face numerous engineering constraints such as the cost/reliability/weight/volume limits of the system.