I’ve ridden just about everything there is to ride on Arran.
Best stuff is;
The routes around Brodick/Lamlash
Ride/Hikeabike up to the shoulder on Goat Fell and back down again.
Laggan route
Coire an lochan is a nice push up blast down too but not very long.
Probably best to avoid the Clauchan route as it is normally a bit of a bog at the best of times and they’ve been felling and making a mess of the track.
Glen Rosa is an ok out and back with a few little technical challanges but its nothing to shout about IMO.
If you run out of stuff on the island, you can also get the ferry over to Kintyre with the bike for the day and make a couple of reasonable routes using the kintyre way.