Everything Househusband said really.
A normal road jersey will easily carry everything you ever need.
2 bottles on the frame to be refilled en route if riding for a long time
Park tyre lever, small penknife, zip ties and smallest Topeak pump velcroed together
Small plastic Park box which a puncture repair kit came in filled with random spares (all bolts, spare cable, links, etc)
Topeak multi tool with chainbreaker
Park Super Patches and house key slotted inside the neoprene case of the multi tool
Inner tubes (can also be taped under the saddle in drier weather)
Phone in a watertight zip bag some Castelli arm warmers came in
£10 note and bank card in phone case
More than enough stuff for a bimble on the road an much less obtrusive in jersey pockets than it sounds. Nothing has ever fallen out on the MTB or road.
The advantage of a saddle bag is that your multi-tool, tube, levers etc. are always fitted on the bike so you're not (wasting riding time) looking for them before you go.
Conversely, putting everything in jersey pockets means you have to check everything properly before you leave and you avoid assuming everything is in a saddle bag only to find the one thing you need is missing because you removed it the night before when getting the bike ready for the next day