Doncaster is really a pretty small town linked to half a dozen or so expanded villages through sprawl. The position of the river and railways means getting into or through town at rush hour is a pain due to the pinch points, so while Premier Inn would be fine for rush hours as you’re working against traffic flow, if you pick any of the newer chains round the lakeside/M18J3 area it’ll be quicker to use the motorway to hop round to Armthorpe/West Moor/M18J4. Don’t think any of the other cheaper chains of hotels have a gym, or even any of the older hotels, Danum/Regent/Earl/StLeger.Can’t be 100% as I don’t stay in them as I live here.
The motocross park is at J4, and there is riding to be had at Sandall Beat woods and around close to your site. There’s actually lots to ride, just that the lack of height makes it all a bit meh. Enough bridleways and quiet roads to link up the various woods and former mines to make enough to ride from the door, plus the stuff you’ve seen above at the Don Gorge/Steetly’s quarry. East and south of town you’re into drained flatlands.