If you go on a trip that is primarily for business, and the destination is within the US, the full cost of travel to and from the destination is deductible even though not all of your time was spent on business. Meals and entertainment expenses on the days primarily related to business are 50% deductible. These expenses and any other expenses on days not primarily related to business are not deductible. If another person accompanies you for non-business purposes, that person’s expenses are not deductible. The rules are a little bit different when travel is to a destination outside the US.
The per diem rates for travel can generally be used to substantiate properly documented business travel, but not if you own 10% or more of the stock of the paying corporation or are a sole proprietor or partner.
In those cases, you must substantiate the expense with actual receipts, although reimbursement can be made at the standard per diem rate, and will be excludable from income under an accountable reimbursement plan.