Zanzibar or Kenya coast? For low-key, relaxed beach time

Don't want to confuse you by giving you more options but there's also Pemba Island which is part of the Zanzibar archipelago which is much quieter and some say more beautiful. I didn't go there but it's another option for you. I would say Zanzibar is much more similar to Asia's beaches than Mombasa (Kenyan coast), but I can't say much about Lamu because I didn't go.

The main beach on Lamu is at Shela village, about 40 mins walk from the town.
It's miles long and soft sand, but no shade as the beach is backed by dunes.

Zanzibar is quite densely populated (most noticeable in the afternoon when schools finish and the roads are literally lined with thousands of school kids on the their way home and you wonder where they all live), but a) locals don't crowd the beaches, and b) there is so much beach that you wouldn't even notice if they did! Some of the northern beaches can get very busy with tourists, especially Kendwa and Nungwi, but even so it will never feel crowded as in European Mediterranean crowded or anything like that. If you go to the south east (Bwejuu, Paje, Jambiani), you will find long stretches of beach without a person in sight. Bwejuu is probably the quietest of the three.
As for the tides - true, it does go out a long way and all along the east coast you won't be able to swim when it's out. It's not muddy or smelly - the sand is so fine that it may feel like mud between your toes, though. There is quite a bit of seaweed, which is harvested by the locals.
Stonetown, even though it does get crowded, is a fascinating town, definetely worth a couple of days.
Of the other places you mention, I have only been to Thailand, so I can't really compare, but I love Zanzibar. Just be aware that prices in East Africa are NOT like SEA, so even a basic bungalow on the beach may be more expensive than you expect.

Stefo, thanks for the update, and everyone else on this thread, too!
When you say Zanzibar is not like SEA, is it just about the prices of accommodation, or prices in general? What about the general "feel" of the place?

Everything considered for tourists is quite expensive, that inludes accomodation, but also food and drink and transport. It is still possible to eat relatively cheaply in local restaurants, but especially in the more remote beach areas you don't really have that choice.
The general feel of the place really depends on where you are: There are beaches which have a strong honeymoony feel with luxury resorts, then there are those catering mainly to European all inclusive visitors, then there are Nungwi and Kendwa in the north, which in addition to the groups mentioned above attract a bit of a young-ish party scene, with regular beach parties, and then there is the south-east, which still has a bit of a rasta - hippy feel. Paje and Jambiani have also the the last couple of years become quite popular with kite surfers.

rasta-hippy feel, while I'm not one of those people myself, these sort of places are usually, IMO, the best balance between "unspoilt/free of package" tours and "a bit of low-key entertainment is available". Which beaches are like that specifically?

Bwejuu, Paje, Jambiani

Bwejuu, Paje, Jambiani

ah ok, thanks, so it's the beaches in south-east in general then I guess. I'll probably spend half my time there, the other half in "party" areas in the north (Nungwi). Or one of the 3 you mention + Kiwengwa.

Kiwengwa was the first beach catering mainly to (Italian) all-inclusive holiday makers, which beach discos etc.. It was the first place with long rows of souvenir shops along the beach (which have recently appeared in Nungwi and Kendwa as well), at a time, when Kendwa was little more than two or three budget resorts. I have not been there since 2004 (to Kiwengwa, that is), because I was so shocked then. Maybe now, with many beaches in Zanzibar being more developed and going the way Kiwengwa was 10 years ago, I wouldn't be so surprised, who knows, maybe it is even one of the quieter beaches now.
I still like Kendwa most of the "party" beaches.
Don't forget Stonetown, though. Even if it's mainly beach time you are after, it would be a shame to come all the way to Zanzibar and not to spend at least a couple of days exploring Stonetown.

Invaluable insider info Stefo!
Legend
Thanks