Anything worth doing, you can accomplish with 1 or 2 dumbbells. Preferably one, as few body movements are "symmetric." Body does things one side at a time.
Definitely dumbbell(s).
I do also own a good treadmill - but mainly for the convenience of my walking days (3/wk). I run 4 days/wk and do dumbbells and walking the other 3.
I'm gonna say one of the best pieces of cross training equipment for a runner is the elliptical machine. Great machine to add to your training for those of us who are more frail with the mileage, plus a great way to maintain basic aerobic fitness when an injury keeps you away from the high impact of running
I prefer the arc trainer - the movement is a bit closer to running and it just feels more natural. I can also get my heart rate higher without my legs hurting more than they should.
This post was edited 45 seconds after it was posted.
Assuming you can run outside and don't need indoor crosstraining, I recommend something similar to the Powertec Basic Trainer in addition to your running, for upper body. I got one at Christmas for $500. I entered all calisthenics you can do into AI and it said it was a very well rounded upper body workout.
leg raises
dips
reverse push ups (pulling)
chin ups
pull ups
...then on the ground regular push ups and sit ups.
I don't want to get huge and still focus on running "thin", but 10-15 minutes after run has really improved upper strength and tone.