Park at Antrim Castle Grounds, accessible and signposted from Randalstown Road, or at Antrim Forum Leisure Centre, accessible from the Dublin Road, both on the edge of town.
Antrim town itself, perhaps surprisingly, has some fun if flat routes, with lots of singletrack and forest paths. If you are local, you may already know most of these places. Suggested route:
Park in Antrim Castle Grounds, perhaps taking a spin round the Castle Grounds first (where there are a few neat ramps and drops), or at the Antrim Forum, and take the river cycle/walking path to the Lough Shore.
Follow the car park round the lake, and head into Rea's Wood along the gravel path. After about a mile, at the clearing on the right with the pile of stones, turn right for some twisting singletrack (and muck!) through the trees, or keep going for a shorter extension of the path you’re on. Keep your pace up and this is great fun, but tiring!
Head back up to the main road along the path past the old demolished DeerPark hotel, and straight across the Dublin Road onto Kirby's Lane. Follow the road until you come to Kirby's Lane car park on your left. Go down to the car park and you have a couple of options:
Turn left and follow the river path back into Antrim, or turn right and follow the path towards Antrim Technology Park. If you’ve taken the latter option, just after the big bridge you can head up left to the road to nip into the trees then on the right and tackle the ridge that runs high above the river all the way to the main airport road junction with the Belfast Road, or take a spin round the technology park and head back to the car.