Can I write off meal expenses on day-trips?

Unfortunately, no.  Meal expenses can only be written off when you have trips that incur legitimate rest periods (i.e. trips with layovers).  When you are on a trip which includes at least one layover, you can write off M&IE expenses for every day of that trip, including the last day where you return to your domicile.  On the last day, the per diem rate used to determine your M&IE expenses is actually the rate for the previous day.  In other words, you do get M&IE on the last day, but the per diem rate used is not the rate of your domicile.  It is the rate of whatever layover you had the day before.

Some people question if a long sit at an outstation airport constitutes rest.  According to the IRS, it does not.  This IRS link shows and example of what constitutes travel away from home.

Day trips do not get figured into the per diem calculation because they do not include a legitimate rest period as defined by the IRS.  Any per diem received during day trips from your company would be taxable per diem, which means you it is treated as salary (not a per diem reimbursement).  Taxable per diem gets added into Box 1 of the W-2 form.