Some great ideas have been given. I have been making infusions (coffee plunger and boiling water) but only adding it to the last few litres of beer at bottling (I bulk prime). This way you are not stuck with 20 litres of mistake. For example, yesterday I bottled a kolsch. To the last 5 litres I added an infusion of 3 gms of lemon myrtle, smelt great (my coffee this morning is a bit lemony
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).
If you keg you still can do a few bottles with the last few litres. I have tried chilli, basil, ginger, coffee etc. This year I intend to experiment with native Australian ingredients.
There is no reason not to brew 20 litres of a base beer and divide it into 4 or 5 lots at bottling and add different infusions to each. You can then do full batches of the "winners". Infusions at bottling seem to give a more definate flavour and aroma than do ingredients added at in the boil IMHO. Please note, do not add fermentable additions at bottling unless their carbonating effects have been calculated or bottle bombs will result.
Hope this helps.