It may already be the end of July, but it’s not too late to plant a few salad leaves for this season. Unlike a lot of vegetables, which really do need the whole of the summer to produce a reasonable crop, loose salad leaves are sufficiently fast-cropping to be worth planting in July or even August.
Rocket germinates very fast and is worth planting at nearly any time of year. Throw a few rocket seeds into a pot or into the ground, cover lightly with soil and water in well, and you should start to see seedlings within a week or two. Rocket is actually better started either well after midsummer (so, about now) or well before it (early spring), as around June it will bolt (run to seed) much faster.
Leaf lettuces (lettuces that grow lots of single leaves rather than forming a ‘head’) are a better bet than headed lettuces for late planting, as you can start picking leaves as soon as there are a handful of true leaves on the plant. Lollo rosso is one popular option; as is royal oakleaf. Real Seeds sell ‘Bronze Arrowhead’ oakleaf lettuce seeds. I’ve had great success growing these lettuces at all times of year, and they taste great; however, they do take a while to germinate.
Most lettuces, helpfully, are at least a bit frost-hardy, so you can expect them to keep cropping well into the autumn. You can extend this further by building a cold frame. Last year I had rocket and bronze arrowhead lettuce growing throughout the winter, even when it snowed. The plants outside the cold frame survived, but didn’t grow any new leaves until the spring.
Finally, if you have any pea seeds left over, or can get hold of some, they’re also worth planting late. Some mange tout may yet produce a proper crop (experiment!), but at the very least, you can harvest and eat the pea tops as salad.