Pep up your alpines
Plants with a carpet-like growth habit, e.g. some alpines, can become patchy, with central areas dying off. These patches can be in-filled with gritty compost, to encourage re-growth.
Feed late-flowering perennials
Some late-flowering border perennials may benefit from a quick-acting feed before they come into bloom, especially if the soil is not very fertile.
Save seeds for next year
Start collecting seed from plants you want to grow next year, especially hardy annuals such as Calendula, poppy, and love-in-a-mist.
Take semi-ripe cuttings
Take semi-ripe cuttings of shrubs such as Choisya, Hydrangea, and Philadelphus. Root them in pots of gritty compost in a cold frame or even with a plastic bag tied over them.
Get more clematis for free
Clematis can be propagated by taking internodal cuttings (i.e. taking stem sections above and below a leaf, and not cutting the stem immediately below a leaf joint).