If you end up at scunthorpe just get on the next eastbound 185 and get off at the next station Barnetby, as it is better.

Scunthorpe is a lively staion in spotting terms.
There is a good selection of freight through the station. The staple traffic is EWSR Class 60's and Class 66's and Freightliner Class 66's.
The main traffic through the station is steel into the Corus works and imported coal from Immingham on it's way to the Yorkshire power stations. There is also much other traffic including the legendary Household refuse 'binliner' thrice daily from Manchester, Tanks (power station short rakes and heavy tanks daily from Jarrow), Enterprise and Heavy steel from Tinsley.
The best time to visit is arguably on weekday evenings from around 1600 to 2030.
Traffic also uses the backlines behind the station, but is still clearly visible.
Scunthorpe is easy to get to from the M180 (take the M181). The station is simple to get to by road (just head for the town centre) and has a frequent passenger service. This comprises of an hourly service to Cleethorpes and Manchester Airport, operated by First Transpennine Express (Class 158's) and an hourly Northern service to Lincoln (usually 142/153).
Unfortunately you have a 5-10 minute walk into the town centre for food (nearest chippie)! However, there is a high quality restaurant and a place for alcoholic drinks left pretty much straight outside the station in the form of 'Smokey Joe's' and the 'Bondz' bar. Another alternative is left out of the station and over 'Howdens hill'. Take the alleyway and there is a newsagent offering basic 'supplies'.
Trains from Immingham to Corus Scunthorpe do not pass through the station, but there are ways this can be rectified.
A 10 minute walk right out of the station along Station Road and a right turn after Netto will provide access for viewing of the Imported coal to the steelworks from immingham (EWS Class 66's). The class 08 may also be visible. However, you do not see the Northern Trains here.
It is worth noting that there is no suitable vantage point for viewing the Iron Ore trains within Scunthorpe itself. Barnetby would be most suitable for this.
However there is a continuus flow of traffic that would not be seen through Barnetby that terminates at Corus Scunthorpe comprising of Steel flows. The same applies to Donacaster. Much through Scunthorpe avoids Doncaster (indeed Scunthorpe is, believe it or not, actually busier for the amount of traffic).
The Appleby-Frodingham railway society may also be of interest to enthusiasts. They operate tours of the vast Corus plant (home of the most powerful industrial loco's in the country) and brake van trips using industruial steam and diesel traction and class 108 DMU vehicles. See Brigg Tourist board for this.
Scunthorpe station and the yard mentioned are suitable for photography as is a bridge over the line at Kingsway. Althorpe station (3 miles from Scunthorpe) is also good for photography if having enough time in the area, the King George 4 bridge providing an interesting setting.
Above all, Scunthorpe is often overlooked for 'spotting' purposes, being located in between Doncaster and Barnetby, but it has much to offer the enthusiast.