The least expensive option might be a wider range cassette and/or a smaller inner chainring. That will probably call for a new chain, and may also require a new derailleur with more take-up capacity (longer cage).

Beyond that you can switch to a triple crankset, which will probably entail a new front derailleur (even if you don't change the cassette you'll also need a rear derailleur).

Step one is to use a gear chart like this one which will give you an idea of what sprockets and chainrings you'll need to get the gearing you want.

IMO- if it comes down to staying 2x8 with a cassette larger than 28 or 30 teeth, or going triple with a smaller cassette, say a 26t I'd spend the extra dough for the extra utility it'll give you. If you go with a very wide 8s cassette, you'll lose some of the middle gears where you do most of your riding. Going with a triple, lets you keep a cassette with more mid-range gears, while having the granny to give you the low range you want.