To be able to brew on a whim as I think you are trying to get at:
1. Dry malt extract (pilsen or pale)
2. Pick 3-4 specialty malt(s) or adjunct(s) to keep on hand in 1 -5 lbs.
a. Dark Munich or a caramunich,
b. flaked maize,
c. a biscuit malt of choice (victory)
d. dextrin malt (carapils or carafoam)
f. Wheat malt of choice
3. Dual Purpose hops (definitely get a vacuum sealer) - You'll have to fins what you like best... Here are some I use:
a. East Kent Golding (personal favorite),
b. Northern Brewer
c. Cascade
d. Phoenix.
f. Tettnanger
g. Mosaic
4. Choose a workhorse dry yeast and keep several packs on hand! - Lots of folks like US-5. I prefer Nottingham and 34/70. - A neutral Dry yeast strain makes wonderful beer! With US-5 and Notty you can use ferm temps to achieve slightly different flavors.
From there you can make an endless array of beers at a whim, when you're feeling spunky or find a few moments! - I highly recommend it as I got so sick of planning and scrapping brew sessions (due to whatever comes up) and then missing out on time I could have brewed because I didn't have the ingredients on-hand.
- I have honestly found some of my most simple beers to be my best beers.
(I owe it to the folks here always talking me down on the specialty malt additions!)
You can continue to do as you've been doing "to schedule a brew session" where you are obtaining very specific ingredients to make a very specific beer. Then whatever is left over can eventually work its way into one of those "at whim" sessions.
BREW ON BUDDY!