Hi Joanne
Speaking from my experience (I don't do any consulting, but have worked in smaller businesses), I would say it's important to have a sound grasp of financial accounting, since this is likely to be where all small businesses use Excel at least to some degree. I don't mean 'become an accountant', but you should be familiar with balance sheets, profit and loss statements and definitely trial balances (which feed BS/P&L) and how debits and credits work.
More Excel-focused advice: good understanding of pivot tables and charts (as these impress everybody who have never used them before), and make sure you know your way around MSQuery. A lot of places will use an accounting package, but may not be so good at getting the information from the systems (reporting functions in alot of smaller packages are really abysmal). Thus, being able to extract from a relational Db (ie the underlying accounting package) into Excel is very useful.
In this vein, I would say familiarise yourself to a large degree with MS Access too, since many places may rely on this for many of their business systems (which will presumably then be bespoke, and with only limited support).
Hope this helps!
Richard