SkyMiles redemptions are generally linked to cash, so it can be tough to find a "sweet spot". I believe short-haul basic economy tends to be a sweet spot (e.g. 14,000 miles RT for BOS-LGA, or 11800 with a Delta Amex card). Foreign carriers in markets that Delta does not operate in (e.g. KE HND-GMP) tend to be good value for your miles.

The baggage service guarantee that your bags will be at the baggage claim in 20 minutes or you will get 2500 miles is also a way to get a decent number of miles – essentially making that BOS-LGA roundtrip cost 6800 SkyMiles.