If a 128 GB upgrade would be enough for now, and you don't regularly use your SD card slot, then a 128 GB Micro SDXC card plus a suitable adapter is probably the most cost-effective and convenient option. If you need more than 128 GB, buy 128 GB and wait for bigger cards to get cheaper ;-)
Another option would be to use a wireless external hard drive, some of which have rechargeable batteries so you can use them on the move - this is one more gadget to carry around, but doesn't need you to have anything physically plugged in to your MacBook.
An alternative approach, if it's music and photos or video that are taking up the space, is to look into how you could use cloud services such as iTunes Match or Apple Music, iCloud Photo Library, Flickr etc. to store your music and photos, and then keep only a subset actually on your device - possibly at lower bit rate or resolution than the originals to reduce the size.