Planar tranformer
Of course you can design planar transformers for a few watt, e.g using ER9.5 core. You have to find out the specific advantages and disadvantages yourself. I remember that Ferroxcube had some application notes, but basically, you should be able to calculate all relevant parameters, as for wire wound transformers. Small planar transformers make sense, when you can integrate them in an existing PCB, but unfortunately, the stackup requirements are rather different for standard PCB and planar transformers. For low power and signal transformers, the performance drawbacks (e.g. low fill factor) can be acceptable.