If you have height in your garden, such as some steps or a sharp gradient, then take advantage of it. Build yourself some self-watering planters (SWP) and link them all up. Connect the overflow of one to the input of the next and so on. This way, you can water your plants all at once. The last overflow can go to your rainbarrel.
Make sure your overflows are at least the same diameter as your inputs, a point that's often overlooked in SWP diagrams. You can chain these up as above, or like Pascal's triangle, or as a spiral. Any way you like.