There is much discussion that takes place about how to make various bicycles even better. Conversations will involve subjects of transmissions, wheel sizes, gear carrying systems, handlebars, seats and more.
There is no "best thing" to do. Each rider needs to think about what they like, what type of riding they want to do, how much trouble or money they want to expend.
Tires: I like to change things and explore what makes the best bike for me. I see around me a roundup of different tire styles and sizes. I can tell from my inventory, that I like "cross terrain" style tires. Maxxis Holy Rollers. Kenda Krad. Kenda Small Block 8 are the primary ones. I also have some smaller Kenda Kwest Tires, as well as the same tire that is labled "Dahon Special Edition". These smaller tires are 1.35" wide, take higher pressures and are generally a bit lighter than the more "BMX" knobby/park/cross terrain styles which run in sizes generally 1.75", 1.95", 2.125" widths (these are all 20" ISO 406 sizes)
Racks: Generally there are 3 rear racks that we keep in stock for our popular 20" bikes. The Arclite, Traveller and Ultimate rear racks by Dahon. We sell Arclites most, part because of price, but also because of compactness. The search is always a bit difficult for Front Racks. Right now I have a good Sunlite version which is a great platform for items that sit "atop" it. Its not really a side load rack, as the Arclite rear racks are not as well. If you want to go with Panniers, the taller Traveller and Ultimate Racks are the ones to go with. Capacity vs weight is the main contrast. The Ultimate will carry more but it is a bit heavier than the Traveller.
Fenders: We offer the SKS Fender package provided by Dahon. We have both 16" and 20" versions in stock. These are Black Fenders, well made by the German company SKS. We may run down some metal or aluminum fenders at some point in the future, but not just now. We may also get Silver colored ones like the ones we now offer. Not sure it is worth the trouble. A longer "rubber extension" is on the short list. One thing I do find is that if a fender it is too long it is difficult to pop the bike up on one wheel and roll it around in tight quarters. The current fender works quite well that way.
Seatpost (Air) Pumps and Compact Pumps: One or the other, that is all I can say. Seatpost pumps are a bit heavier than some of the Stock Seatposts. Lightweight compact pumps work great with a 16" or 20" wheel. You generally will keep them in a bag or mount their carrier to interface with your bottle rack. We have a good selection of both styles. The pure logic of having a seatpost pump is really hard to beat. Never forgotten and very effective.
Full conversions can be done in the following categories:
Transmission and Transmission Parts
Handgrips and Handlebars
Seats and Seatposts