I’m no expert on the Lakes but done a bit of riding around Ullswater.
Along the Eastern shore of the Lake towards Pooley Bridge, back via Howtown and up to Boredale Hause then down an excellent very techy descent is a good short option. Easily combined with the route below for a really big day out.
Did a brilliant ride up Helvellyn about a month ago – up Keppel Cove (bit of pushing but not much) and down Sticks Pass, then up Keppel again and down Dollywagon. The first descent is nice singletrack with loose switchbacks to finish. The second is pretty epic, starting with steps and drainage ditches which aren’t for everyone, but then amazing technical fun that seems to go on forever. Write up, pics and vids here so you can see what it’s like:
A Jolly Down Dolly’
Could be altered slightly if you go down Birk Side rather than Sticks to avoid repeating the climb.
Other options would be up from Hartsop, along the High Street, down a fast grassy descent then back along Ullswater the other way. It’s a while since I’ve done that so sketchy on the details.
Or you could drive to Kentmere and do the Sadgill, Gatescarth, Nan Bield ride. Not many miles, but lots of climbing and carrying. The Nan Bield descent is very tough but very good.
Enjoy!