We all have our own motivations and each of them is as valid as the next one. You are, after all, an independent contractor selecting those jobs that work within your parameters.
One word of caution. If you approach this as a 'hobby', which you may readily want to do since you are primarily interested in the entertainment factor which is mostly reimbursements, any fees you earn are directly taxable to you as income without deductions. If you approach this as a 'business' you will need to file a Schedule C with your tax return and keep appropriate business records. You then become eligible for mileage and other deductions against your shopping income. My best guess is if you are traveling on business your mileage is already covered by your employer as is, perhaps, a per diem. Claiming income as 'hobby' income is probably a better option for you to avoid 'double dipping'. Reimbursements are not taxable, but if you are being reimbursed for the shop and by your employer under a per diem I would talk with my tax guy/gal about whether one of the reimbursements constituted 'income'.