Use a hope chest as a window seat. These are strong and sturdy and will easily hold one or two people sitting down. Place a tapestry or a small throw rug over the top and add a couple of pillows.
Use an outdoor sitting bench. An upholstered bench works best, although a bench with baskets underneath and seat cushions on top can work great also. Cushions are important in creating a comfy window seat, and both decorative quilts or tapestries can be draped over the back.
Place two pit group ottomans side by side next to the window. They are already fully cushioned and are great for lounging on.
Consider a daybed for your seat. Use a daybed or a fainting couch for your window seat. Place either under a window and pile on a lot of colorful pillows.
Use a tack trunk for country homes or for a more rustic look. They can be dressed up by throwing a horse blanket over the top, or leave them "as is" to show off the hinges, latches and handles. Tack trunks will easily support the weight of two people.