Can you walk around alone or is it worth getting a guide? How long to allot to visit? Any restaurant suggestions?
Never been to Dinan but the highlight of St Malo is the beach and watching the tide roll in in the evening.
I did not find that I needed a guide there.
There are town walks for both in the RS France guidebook. Walking the walls in St Malo is interesting. It has a different feel and architecture than many of the other towns in Brittany with the half timber buildings.
We have stayed in Dinan twice and day tripped to St. Malo. Both towns are easily walkable and Rick's walking guides are fine. His tip on parking in St. Malo at the Vincent Gate is excellent. We like the restaurants along the old port In Dinan, (Chez Bongrain was one that comes to mind) good food, great setting and fair prices. In St. Malo we had our lunch/afternoon snack at the Creperie des Lutins on the Grand Rue, you can check it out on Google Maps ... good food and unreal atmosphere.
The hard lesson we learned on our first try at St. Malo is don't do it on a Sunday afternoon. That is when it seems like half of England is lining up to get to the ferry piers to go home. Traffic was a mess.
Thanks all for suggestions!
I'd like to add a caveat concerning the "both towns are easily walkable" statement. St Malo is indeed easily walkable; Dinan upper town is indeed easily walkable; Dinan port is indeed easily walkable. Going from the port to the upper town in Dinan is not what I would call "easily" walkable! We spent a week in a charming apartment in the middle of Rue Petit Fort, which connects the upper & lower towns in Dinan. Both are charming, connected by the medieval, cobbled street, lined with 500 year old houses. The hike from one to the other, though far from impossible, was somewhat daunting. By the end of the first day we learned to plan our trips up or down to minimize duplicating the climb. If I left the camera at home or forgot to buy a baguette when I was out, it was not trivial to run out & fetch it. The street is far steeper than anything allowed by zoning today & the surface of ancient pavers is, of course highly irregular. I should also note that cobbles worn smooth by 500 years of foot traffic become very slippery in even light rainfall. In our week there, we saw folks take falls on 2 occaissions.
All of that said....Dinan was a delight & we loved our time there. Both the upper & lower towns were damn near perfect & it proved a good base to day-trip into Normandy & Brittany. Just keep in mind that visits between the two halves of Dinan should take some consideration, especially for the mobility impaired.
I don't think you need a guide. St. Malo is a neat place with a complete wall and a little beach. Walking out to the island fortifications at low tide is a treat. Inside the walls, it's pretty much tourist trade, at least on the ground floor, shops, hotels and restaurants. There is also a museum but I didn't go when I was there. I arrived early, maybe 9 am and left after a fairly expensive crepe lunch (with a good view from the wall). That was at Creperie le Corps de Garde, the building is worth some of the price of lunch. If you aren't stopping in the museum, laying out on the beach or doing a lot of shopping, you probably need a half day.
For Dinan, Rick has a self-guided walk and Michelin Green Guide has a nearly identical self guided walk through the center. I liked the handful of restaurants down on the port. One was recommended to me but it was closed the night I was there (they open more days in high season) so I tried another and had some very nice Moules Bretton with frites and a beer - not expensive. I would say a half day to walk the center plus time for a meal and a walk around the port area.