I almost always have a piano. But the best exercise for my fingers when away from it, is "playing" piano exercises on my leg. Position your hands so that they are comfortably over your body surface. Bend your elbows to a comfortable angle. When standing, my front pants pockets are in the right place for me.
Hanon exercises are great. All you need is the fingering. For example, the first exercise is simply 1 2 3 4|5 4 3 2|1 2 3 4|5 4 . . . and so on.
Another good one is 5 3 4 2|3 1 4 2|5 3 4 2|3 1 4 2 . . .
And the reverse: 1 3 2 4|3 5 2 4|1 3 2 4|3 5 2 4 . . .
Go slowly with some force behind it. Go fast with less force. Go evenly with the metronome in your head. Or a real metronome if you have it.
Be sure to stretch your forearms as needed during your workout.
In addition to the "playing"/tapping above, spread your fingers out wide on your pants or shirt, grip lightly and close your hand against the light resistance. keep your fingers spread while closing. Sometimes close fingers in sequence from little to forefinger and reverse. Similar to milking a cow.