Build a cheap 'cross/road/ss/fixed – up from the bits in your shed/someone elses shed.
Get cross tyres or skinny 26-ers that roll nicely.
Ride to work the long way (different routes all the time)
Ride home the long way.
Weekends and longer evenings – put together some routes that invovle small country lanes/hardpack green lanes/bridleways/cyclepaths/towpaths/
Do big loops,all over the place.
Get your mtb out and ride the nearest city/town/village/urban area :- play on steps,street furnature,parks, small backlanes – link them together with local cyclepaths. You will find stuff all over the place.
Get some rollers and practice riding one legged,one handed,no handed…..