As said : fuel expensive and has to be decanted from jerry cans unless you arrange for a gravity feed from a IBC from the fuel suppliers...ie 100 litres plus
Bryher has good anchorage for ribs close in and also plenty of moorings.
St Martins campsite is right on a lovely flat expansive beach which can be anchored on but you'd have to get the boat off at 2 hours before till 2 after LW if you want to use it over that period.
Be wary of weather conditions as they can change quickly with steep short waves in tide against wind conditions making ribbing "exciting" to say the least.
Know how to read a chart/plotter well and know the tidal ranges as very often i see evidence of boats that "just check that the rock was actually there" or "oh yeah that is a sand bank"
Apart from that its a great place for exploring and beaching for picnics.
I'd suggest in your trip: picnic on little arthur (half tide ebb) reveals a lovely little beach in a cove Nice in easterly wind as its sheltered.
beach the rib on ganilly bar at LW for a play on a sand bar.
Appletree bay or pentle bay (both tresco) at HW for water sking dependant on wind direction
Samson for a evening BBQ and watch the sunset from the top (i didn't say that bit as no naked flames but use your descretion)
When the weather is good (no sign of fog) and fuelled up with good safety kit...trip out to the bishop rock lighthouse (6nm) and then back in through the western rocks ....You'll need to be confident in your nav skills for this bit......past the gilstone (site of the association wreck,rosevear where the men lived when they built the lighthouse -you can still see the gable ends, plenty of seals,puffins and usually dolphins,sharks etc)
I'm biased as i live here but its a great place to own a rib.
so if you get over then say hello if you see a blue ribcraft 6.8 with a suzuki 250 on it.
If your stuck don't hesitate to ask
if you get stuck the rib can be transported back to penzance by ship (sc3 or gry)