I would second Hilberry. Top tip would be to take a packed lunch as the burger van is pretty expensive up there.

For Saturday, public transport only and weather dependent, possibly non-bikers might be biked out.

Suggestions

Steam Train to Port Erin and chips/ice cream/pints in the Bay.
Manx Museum is worth a visit and its free
Camera Obscura is on Douglas head
Electric railway to Laxey and up Snaefell or onto Ramsey
You'll be able to get the bus to Peel to the House of Mannanan, Peel castle, plus the pubs are good, kipper house and I believe there is a small transport museum there.
Bus to Castletown, there is the Castle there but not much else. If you ask the bus driver to drop you at the Aviation Museum, its interesting too (depends what you like though)

or you could just go to the pub