I've used the Patagonia Refugio 30L for a couple of budget trips, and it's been a solid choice. It's versatile enough to serve as both a carry-on and a daypack. The size is manageable, and it generally fits under airplane seats, though it can be a tight squeeze if packed to the brim.
In terms of durability, the material is quite robust—it's made from recycled polyester and has a water-repellent finish, which is great for unexpected weather. Comfort-wise, the shoulder straps are padded and supportive, making it relatively comfortable for long days of exploring. However, because it's a more basic backpack, it lacks some of the ergonomic features found in specialized hiking packs.
An alternative to consider might be the Osprey Daylite Plus. It’s also very popular among budget travelers for its comfort and adaptability. It’s slightly smaller but has excellent support and ventilation features.
In any case, think about packing light to ensure comfort and avoid any issues with airlines' carry-on policies. Happy travels!