Since you're doing it yourself, the answer to the topcoat is is polyurethane. More durable industrial coatings exist, but they are difficult to buy and apply.

The stain type is less important. Try and get a scrap of the mahogany from whoever made the bench, so you can practice getting the color right.

Thank you so much for your reply. I've got the scraps covered - and good thing too because I tried what I thought would be the right color stain, and it wasn't...

I tried doing a little reading last night on finishes, what about a "penetrating resin"? My husband thought a "hard coating" like a polyurethane might scratch/chip easily. Is a penetrating resin not enough of a "sealer" that I could end up with watermarks/stains instead?

Thanks!!

Hmm. "Penetrating resin" is new terminology to me. It doesn't even show up in the index of Bob Flexner's rather comprehensive "Understanding Wood Finishing." A bit of Googling suggests that it can mean anything from wiping varnish to thin epoxy. For your purposes, it would probably be the former, since the latter is not a finish at all. Such finishes are generally easier to apply and they do look nice, but until you build up a film of resin on the surface of the wood, the protection they provide doesn't amount to much. To protect something that's going to take serious abuse you need that film finish, which is what polyurethane accomplishes in just a couple of coats. To achieve a similar film with a wiping varnish, you'll be applying many more coats and the finish will take longer to cure. Either will work. There isn't necessarily much difference in the finished product.

In my experience, chipping isn't much of an issue if the finish is applied correctly to begin with. Make sure you don't leave excess stain on the surface. Make sure the stain is thoroughly dry before applying the topcoat. Follow the directions for timing, temperature and abrasion between coats. Let the thing cure thoroughly before putting it into service.