Don't rush into things, outside arena work: trails, loping/trotting up hills, trotting in water and or deep sand
Arena work: ground poles, lead changes etc :)
To add to my answer. Keep in mind the reason for the time off also. If it's from injury or just mental vacation.
Start out slow and don't rush anything! Basically what Gypsy said!
It depends on the amount of time off and the reason for the time off.
However, always start slow and keep the amount of time exercised short.
Walking and slow trotting are good to start but keep in mind the first thing I said also.