^ Cosmos are easy, fast growing and really colourful. You just need to follow the instructions on the packet whether you want to sow in pots or straight in the ground. Why not grow some veg, things like rocket, french beans & carrots are really easy and just as attractive as flowers IMO. If I were you I'd spend most time preparing your patch, double dig & compost it now and it should be ready for planting in a couple of months. i'd try and edge it decently to keep the grass out, sink your edging to just over soil height so you can mow over it.
I sowed green manure in late summer and now I need to dig it all in which looks like its gonna be almost as hard work as making the patch in the first place. I get into it once I start, its just getting motivated to go out there in the 1st place. But Ive missed planting seasons for various things the last two years, one thing I've learned is nature doesn't wait lol. I will have sweetcorn this year dammit