Songbird Blend is your go-to if you want a budget-friendly "one feeder" blend with a little something for everyone.
Complete with Black-oil Sunflower, Safflower, Peanut pieces, and Hulled Millet, this blend will offer the most bang for your buck if you want a simple, no-frills opportunity to watch birds on a budget. Ratios of each seed by volume can vary significantly, but all bags will include at least some of each ingredient.
Serve in a hopper, tube, tray, or even just sprinkle it liberally on a windowsill or railing. The only reason not to serve it in a finch feeder is that the feeding ports are too small, otherwise you could do that as well! To limit your offerings to smaller birds, offer in a tube feeder or "clingers only" style mesh or hopper, or open your restaurant to everyone with a platform or other "general seating" equivalent.
The only question here is, who will ask for seconds first!
Note: window feeders and seed on window sills do not generally contribute to window strikes, as birds utilizing these are actively stopping at the food rather than trying to fly "through" to whatever is seen in the reflection.