I’d 2nd the Herm suggestion, folk festival in Sark this weekend will mean the boats will be full as its a sell out.
If you choose not to got to Herm, check out the bus station for a cheap round the island tour, it used to be Route 7 but the routes changed a few weeks ago so i’m not sure of the number or frequency but the guys in the office will help out.
St peter port is worth an exploration, the tourist office will let you know if theres any walking tours on which are always interesting, i’ve lived here for 40+ years and always learn something new. If theres no tour on then just have a mooch around the back streets and lanes.
Terrace Cafe does great al fresco Thai food and a good selection of beer, dix neuf is a bit more up market cafe, Petit Bistro does a good French lunch and the Boathouse does excellent sea food.
Ask anybody for directions and as long as you get a local they’ll help you out.
Castle Cornet is a great way to pass a few hours as well as Victor Hugos House, the German Tunnels at La Vallette will give you an insight into the occupation, more indepth occupation insights can be got at the Occupation Museum in the Forest.
There’s a ship wreck museum at Fort Grey on the west coast which is also worth a visit.
South Coast cliffs are great for a scenic walk but it will take you all day to walk the entire cliffs.
Glorious weather this weekend will mean plenty of people on the beaches, most popular is Cobo but it will get busy, Grandes Rocques is a good second choice as well as Port Soif (great for swimming when the tide is up, great rock pooling when its out)
Pembroke is also a great beach whatever the tide height and theres a good beach cafe there too.
Fermain bay is also a nice beach on the east coast with a good little cafe, can be walked along the cliffs from St Peter Port (after you’ve been to the German Tunnels – its on the way).
You can also rent bikes from a guy near the tourist office.
If you fancy something adventorous check out outdoor guernsey http://www.outdoorguernsey.co.uk/ or Island Rib Voyages.