Dropping 12V to 7.4V
On eBay there are adjustable step-up and step-down converters for a few dollars a piece.

If your mindstorm brick needs exactly 7.4v (which I suspect it doesn't) and your 12v items don't need exactly 12v, then I would definitely use a step-up converter because it could be inexpensive and the adjustment would not have to be critical. It would also be quite efficient (my guess 90%). 

If you need a 12v supply with a tight tolerance, and already have a stable 12v supply, then you could make your own 7.4v regulator quite easily with a circuit as below. The efficiency would be slightly under 7.4/12 = 61%.







simulate this circuit &ndash; Schematic created using CircuitLab

I do not know how much current the Mindstorm draws, but the circuit above should be good to up to about 1A. It would not be too hard to increase with a few changes to the circuit, such as a second NPN transistor, or replacing the bipolar transistor with a MOSFET. (It may be necessary to find an op-amp who's output can get closer to the 12v supply.)

In the circuit above, for R1 and R2 you will want low-drift so that the voltage does not vary too much with age or with temperature etc. Also depending on how accurate you need the voltage, you may want a multi-turn variable resistor to be able to set the value more easily. However, then you may also want to take measures to increase independence of the 7.4v regulator from the 12v supply, as a dip in the 12v supply voltage (e.g. under load) will cause the 7.4v output to drop a little bit. The higher your requirements, the more complicated the circuit. This can be taken to the nth degree.