It largely depends what I'm doing and where I'm riding.
If I'm going to one of the organised mtb events, then I'll take a shed load of stuff along with me in the van. Over and above items listed lower, I'll take along items such as a track pump, various lubes, a box of nuts and bolts, waterproof jacket, change of clothing and a certificate of conformity for the bike. Clearly those items stay with the vehicle, but I will carry two extra inner tubes for the ride. I tend just to use my Camelbak Rogue for these rides.
If I'm riding locally, and expect to just be out for say 25 miles and a couple of hours, then I'll take my Camelbak Rogue, one tube, scabs, Mountain Morph pump, tyre levers, spare chain links, a multi tool with chain splitter, and a few cable ties, some money, a phone and possibly maps.
If I'm going out for a full and lengthy off road ride, then I'll take my Camelbak Mule NV, and added to the above kit, I'll also take along energy foods, a small fold up poncho, a first aid kit, chain lube, which will be either wet or dry depending upon conditions, a few nuts and bolts, and a note pad/pen.
For evening rides, I also take along a small torch, although one of my cheapo multi tools has a built in one.
I also always take along medical and ID data. Hopefully the worst will never happen, but this info doesn't take up any room, and could potentially save a lot of grief.