I live on the coast, not far from Donna Nook / Mablethorpe.
As has been hinted at, both Mablethorpe and Skegness will be rammed on a bank holiday weekend, but will quieten down as the week progresses. It can be a bit rough and ready, as has been said, but it has its charms. The beck in Mablethorpe does a cracking carvery, despite its chav appearance, but I would avoid this Sunday as it will be rammed.
Sutton on Sea is nice; the beach is good and the place is a a bit less run down than Mablethorpe. The Dolphin chippy does a cracking sit down haddock special. The millennium cycleway that runs between Mablethorpe and Sutton is a nice family friendly cycle / long walk.
Donna Nook is pretty in the sun, if you like nature and if you venture that way, the Axe & Clever in North Somercotes is good for food and real ale; a proper country pub, has a play area, although I’m biased as its my boozer. 🙂
Drop me an email if you need any specific local info, I’m sure between me and my mates we can point you in the right direction.
Hope you have fun!